There’s a reason for this.
Up to this point, there have been hidden tracking event quests that could count towards quest-completion Achievements. Those hidden tracking quests could result in a player (who carefully counts what they’ve been doing) receiving a ‘50 quests completed’ Achievement after only counting 44 visible quests.
We’ve made a change in the forthcoming Fractures in Time patch that will put a stop to that. After this patch is released, only quests that you see and accept and complete will count towards quest-completion Achievements. Players won’t lose any progress or lose any Achievements they already have, but from that point on, quest-completion Achievements will be counted correctly from the player’s perspective, and shouldn’t appear to be awarded earlier than expected.