No, that is not normal, it's an unusually high kill rate in my experience. When I've been in parties that hit those levels, there have usually been one or two kills per campaign that require resurrection (though more close saves with resurgences and whatnot).

It may be due to bad player tactics, weak characters, or the GM runs things tougher than the average GM - either "higher CR" or just "better", some GMs (like myself) manage to squeeze a lot more kill per hit die out of opponents. Sometimes a GM runs a harder game, which is fine.

But of course the thing about high level D&D is that you have multiple levels of defense against death, so as long as it's not impacting the fun, there's nothing really wrong with it. You get to play croquet with the angels on a regular basis, heck, that could be part of the plotline (Order of the Stick, anyone?)

If you do want to do something about it, and don't want to embrace character optimization, I would review the results of combats with the rest of the group and see if the deaths were preventable - do party fighters repeatedly charge recklessly into combat, ruining the ability of mages to area effect; do you get spread out all over the battlefield where the cleric can't effectively help you when you're in trouble; do characters not participate well (the tank fighter who hides in the back, the cleric who's too busy fighting to heal)?

Tactics In Depth

I had to coach one player group that was getting demoralized about how tough combats were going in one D&D campaign. In general they didn't have any coordination - they'd open a door and see bad guys. One guy would run in (before the mages could cast damage or battlefield control spells), others would back off, people didn't have mobility or line of sight. They'd decide to run, except most of the group would retreat, but one guy would want just one more round of full attacking, and then be cut off, and then half of the rest of the party would keep fleeing but the other half would come back and help them, and get cut off themselves... I taught them to do simple things like NOT go in the door, but back off and form an inverted triangle around the door and let them come to the party (for dumb melee opponents of course) so that they are the ones getting surrounded and losing actions and being separated. One PC that was taking on a leadership role took it upon themselves to "call the shots" in combat so that a retreat or attack was performed by the whole group. There's a lot of D&D-specific ticky combat tactics stuff, but I always see the greatest difference being made by system-independent "having your crap together" kinds of basic techniques. Have patience, don't get split up, coordinate maneuver to take the fewest attacks but deliver the most, concentrate fire.

I remember in one five year long 2e campaign I ran, the team leader went so far as to run IA drills and basic response plans. When they would come into contact, there were set formations they would move into automatically. The shouted command "Blue" told PCs to close their eyes to avoid the mage's Color Spray. The only PC death in that campaign came from PC on PC action.

Make It Harder

This is definitely something I've had happen. I wrote a whole blog post about the exact same thing - while GMing a fourth level Pathfinder party, I found that I had to make bosses eighth level to challenge the PCs. So you're probably going to need to up your CR/EL expectations. Pathfinder PCs have higher damage output therefore old 3.5e wisdom is somewhat out of whack (same with 3.0 to 3.5 - in general for 3.0 CR X you need 3.5 CR+1 and Pathfinder CR+2). You can see this in the encounters in Pathfinder Adventure Paths as well, the CR of capstone encounters is often like APL+5. So I'm afraid you're going to have to go for some "harder." Also, solo opponents in Pathfinder are meat for the beast because it allows the PCs to exert tactics without resistance; give bosses some help even if they are just a wad of mooks.

Play Harder

Now you can also push opponent effectiveness by improving their tactics and not just beefing them up - but I already do this, and it doesn't completely close the gap. But yes, try to come up with effective tactics for the bad guys, especially if they are on their home turf. No one plays too fair when their life is on the line. Random orcs aren't going to be brilliant but they don't just charge lemminglike into the blender either. Look at their preparations from the point of view of "if I were doing this to be effective." I like recruiting actual people to play main bad guys - they are a good 30% more effective than when your attention is split between running them and doing all your other work. Try some of the player-oriented tactical advice from this question on your NPCs/monsters. In a wilderness setting, people who have any warning will use terrain to their advantage - "Shoot at them from up in the trees," "Get in the canyon so they don't see us from far away," or whatever.

Goal Jiujitsu

Also if you're not having many fights a day, consider that challenge doesn't have to be all combat. What is the party's goal? Unless it's "harvest souls because we are serial killers," it's likely their goals involve finding things, saving things, building things, helping people, or whatnot where the challenge is more skill and roleplay oriented and not solved by killing. For example, in the Kingmaker adventure path, the PCs are in the wilderness trying to build a kingdom, and if they slaughter everyone that faces them then they don't have much of a kingdom once the fights are over!