The metagame is established generally through the thousands of games played. Usually, teams will get together and decide to try a team composition and success will lead them to keep trying it. Obviously, over just a few games everyone's performance will vary widely and no meta will save you on your bad days...
...But over thousands of games with hundreds of thousands of players, individual skill becomes averaged out and what you're left with is what strategies tend to work. Now, we need to distinguish a strategy from a meta to make a lot of sense here. I'll do it very briefly: The metagame is a framework in which you insert individual pieces (champions and builds, where builds are item/mastery/rune combos) that fit the framework. In our current state, the "meta" is usually a high-survivability (but not usually tank) in top who can solo and scales well with items and experience; mid is a character that scales best with experience; bot is a character who scales best with items and needs either baby-sitter or someone to setup kills; the jungler is highly variable, but often is prized for their ganking abilities. So, take any "pieces" that fit inside that framework, and you have a particular strategy within the meta. For instance, an AoE heavy composition is a strategy within the current meta.
Strategies are handed down from pro players (usually, but not always), but the meta is determined by all of us. It's less a "choice" that people get to make and more a mathematical optimization realized over a ridiculous number of games (millions). Metas will usually stabilize without some dramatic changes in terms of patches and champions, which is what has happened in LoL. More casual players will tend to emulate what they see the pro players do, which makes sense in terms of the number of games played per person.
It should first be said that the metagame always has the ability to change at any given time. Several factors impact this heavily, namely success found in tournaments by the competitive professional teams.
As seen at IEM Hannover, the meta game between the three regions is essentially the same. All of them utilize AP mid, ranged AD with support bot, jungler, and either a bruiser top or AP, depending on what team comp they were going for and counterpicking. This has effectively been the metagame in all regions, and until someone comes with a different metagame that does well against it in a tournament setting, it will likely stay.
Prior to Dreamhack, the NA meta (can't say US only, since many players from Canada play, for instance of several members of CLG and TSM are Canadian citizens - at WCG: ChaoX, TheOddOne, HotshotGG, BigFatLP, and Elementz represented Canada) ran quite differently to EU. NA teams at the competitive level ran a tank top, jungle, AP mid, ranged and support bot, while EU teams ran double AP in solo lanes, jungle, ranged and support bot. The casual games on NA was still running, double tank lanes bot, AD carry mid, AP top with jungle, and many games had no jungler. I know this because this is when I started playing LoL heavily. At Dreamhack, it showed that EU was more used to AD with support bot, and great plays were made continually by their bot lanes. The eventual winner of course was fNatic led by xPeke and Shuushei showing off their double AP comp to quickly burst the enemy team with AoE damage and making it easy pickings for their AD to finish everything off.
Flash forward to IEM GamesCom, the NA teams now having chance to get more practice with the AD/support bot lanes and think of ways to counter the double AP strategy. NA teams, come up with tanky DPS top and tanky DPS jungle giving way to the Bruiser meta. In one game of TSM against an EU team I cannot remember, TSM takes Soraka/Jax bot lane to combat the Urgot who had given them problems the game before. They had three bruisers and managed to pull off the win before falling to CLG in the finals where the first two games was playing protect the KogMaw.
To answer your question then, it wasn't really a matter of teams playing different metagames, however they understand what counters what and know how to play accordingly. Double AP gets countered by a bruiser/tanky DPS comp. AoE team comps can win teamfights but can be slaughtered to a poke comp where they don't allow a teamfight to happen before they poke you down too much, a push comp counters a poke comp. So counterpicking, and strategies to execute is used a lot to help gain the advantage. For instance, picking a very strong late game comp against an early game team comp and playing super passive to stall the game. Nasus, Irelia, Karthus, Vayne, KogMaw are examples of champions that have an insane lategame but an early game that can be shut down (not so much in the case of pre-nerf Irelia). On the other hand, Urgot, Ezreal, LeBlanc, Alistar, Udyr start to fall off heavily in the late game compared to their counterparts unless they have built up a big item advantage.
As for everyone knowing and following the meta, it can be both a benefit and negative depending on what side you take. In many tournaments, teams have picked like they were following the meta, but in game swap lanes to give themselves the advantage. This is most common with top and bot lanes switching. It also happens with mid lane and top lane as most players use their runes to give themselves a laning advantage. For example, let's say one team's top and mid are Malphite and Galio respectively. The enemy uses Riven and Karthus. Malphite thinking they will face Riven, choose to use a full armor page to effectively negate most of Riven's harass combined with Malphite's passive. Galio similarly use full magic resist page to negate anything Karthus can do in lane. Lane starts and you find they swapped on you. The difference is they could have chosen to rune for it or not, but either way, Malphite and Galio is at a disadvantage since their runes do little for them. Sure Malphite's damage builds with armor and Galio gets more AP with magic resist, but they aren't going to have much fun in the lanes. To compensate, they try to quickly switch. But the enemy team can switch as well, forcing another switch or unfavorable matchup. Each time you switch lanes though, one of your lanes will tend to fall behind, and since you made the first switch, your team will lose out allowing the other team to gain an advantage.
Best Answer
The optimal composition is 2 sololaners, (usually, one bruiser, and one AP carry), 2 duo-laners( ad carry and support) and one jungler. The bruiser is usually top, the APC mid, and the dual - lane is bottom.
Switching lanes is a metagame, it was something very unusual a few months ago, and now it is quite common.
Metagame changes may come from several things:
These are just a few changes that changed the metagame.
EDIT (thanks vogel): What I wanted to point out is that metagame changes often appear after a game update. Although changes are often initiated by players (mostly top players, to be honnest), but some of them are wanted by Riot (ie: nerfing oracle, to make the game more dynamic and aggressive, giving AP jungler items to make them viable in jungle).