96% Complete?

Hey guys, I have a minor issue. Today I went through Halo 2 on Legendary, and it says the achievement for Heroic and Legendary playthroughs is only at 96%, and I think for Heroic it says 97%.

Why is this? It isn’t the prologue, as I have made sure it isn’t. I know Achievements are buggy right now, but from what I’ve seen, it will appear 100% for people who have completed it.

Any help?