Do you remember if one of the quests was failed? NPC died or similar. It'll show as 85/85 but won't grant the achievement.
I'll check, I'm pretty sure I've always reloaded if an NPC died but might not have some time.
I know for a fact that I for example helped the Varannari in the cases like the Jabal quest and the slavers in Eastern Inori, if that matters. And in the werewolf defense quest some soldiers died, not all though, and not the named woman. And I sent Amarisa away rather than taking her with me. I also returned the medallion in the Spicing Things Up quest to the woman and not the Golden Hand. Hmm.. and I returned Latimer his son's remains rather than lying. I gave the crown to Tol. I let the captain destroy the heart in The Lady of the Night. In Swine Troubles I refused to take part in their squabbles but told Mifadis the truth. In the Snake Gang quest I robbed the Golden Cove bank.
Really the only thing I can think that should affect it if anything is a less than perfect outcome in Defending the Outpost. Because if it's any of the other things then that implies that you really aren't intended to take whatever path you wish, since they are all alternative ways to complete a quest rather than failing it or whatever.