IC2 Achievements reset on reload, a least in SMP, Vanilla are left unchanged.
Fix your game, Dinnerbone!
Normal behavior of achievements and stats. They tend to freak out with mods.
I found a solution on the Technic forums. http://www.technicpack.net/for…/achievement-reset.18864/
It likely won't work with the current version, but it does describe the problem. I was actually considering making a bug notification with that link so a current fix could be made.