Looking at the achievement rarity section on my profile and noticed I had 1 achievement that is being counted as having nothing to it. Not found which it is without going through all of them. Image attached to show what I mean. Clicking on the number just lists all achievements.
Well since Fsh isn't saying it and I'm not qualified to say it, I won't, but hotdogs probably right about a bug and since its been posted here its in the que somewhere so someone should see it and since this is close to the holidays, I'm going to say that it probably wont get looked into until after the new year. I hope you can live with it for a while and sorry for any inconvenience.
I seem to have had this issue occur for me now. While checking through my list of achievements unlocked, I happened to notice one that's possibly causing my problem -- In The Beginning from Pneuma: Breath of Life has 100%, but no rarity category underneath it and when hovering the mouse cursor over the 100%, it says it has been unlocked by 600 of 599 players.