Halo: Reach Medal Collection

This is only a theory, but were the ones that were getting reduced credits xbox live profiles?

If a profile is an online one (AKA a gold subscription I believe) then they work on a different credit scale than that of an offline account. This is done so that offline accounts can experience all the joys of the armory easier, and online accounts can't abuse offline play to max out credits easy.

This is what I'm basing this theory on:

On a similar note, I’ve been seeing some chatter about the prospect of mass quantity credit dumps that can be earned through offline play in various game modes. Some of the laziest among us are no doubt already plotting out their path of least resistance, thinking of ingenious new ways to rubber band a controller to achieve a Zen like state of ridiculous effortlessness.

Not so fast. When you escort your offline profile into the wonderful world of Xbox LIVE, our servers will identify your newly connected status and instantly notify you that our online credit scale operates on a completely different level. (That fizzle sound you’re hearing is probably your boosters running out of fuel in an oxygen starved environment. Don’t look down, RocketMan.)

If you choose to remain offline with your decked out profile, you get to keep your ducats and your duds with no questions asked. But should you opt to take this party online, your credits will be reduced significantly and any Armory items that you had previously earned (for which you no longer meet the minimum online credit or rank requirements) will be righteously revoked.

Of course, the credit reduction that occurs when you make the move into cyberspace is completely by design. For players without a connection to the outside world, the system is simply accelerating the credits being awarded to allow them to experience the Armory at a realistically achievable (but decidedly lonely) offline pace. For those who opt for a more social experience, the grind is a little more protracted, but still profoundly fun.

Source: http://www.bungie.net/News/content.aspx?type=topnews&link=BWU_090310

I don't think they need to be. I just got Offensive Driver on Boneyard.

Edit: You can unlock any achievements apart from the level-specific ones on any map as long as you have downloaded the DLC.