Legends achievement question

I’m just beginning to play through the campaigns on legendary with a friend and have thus far only completed a couple missions of Halo 2 but the achievement shows 52% completion. Any idea why that might be?

Also I’ve heard of people having issues getting the achievement to unlock. Is this very prevalent? I want that Helioskrill armor and would be upset if I go through all this and can’t get it.

Thanks!