Well i just completed Halo 2 on the legendary difficulty and got my achievements as normal. Also the achieveemnt for completing 300 games popped up. When i started looking at my achievement list it seems that i also got the achievements for 350, 400, 450 and 500 completed games for free with it. The Halo 2 campaign has a lot of missions, but not 200 
Just so the developers know, i don’t mind getting free achievements but just wanted to point this out since i don’t want to be counted as a cheater just because the achievement unlocking is bugged…