Ye, I got the same issue, I have completed about 2000 Quests but it'll only display 1200 quests. It's a wierd bugg that came with Cataclysm. Hehe ◕‿◕
Didn't they mention about this pre cata, about the world shattering and new questlines. Im sure they said to get your achievements quick because as they remake all the quest lines, those that are deleted in cata, you will lose all credit for? Prob all this is
Its not a bug. Pre cataclysm you earned those achievments and when all the quests were redone the count dropped to however many quests you have done that remained. You keep the 500 quest achievment because you already attained it.
Nothing fail about that. All quest done in Kalimdor and some in EK where wiped at launch of Cata from your history of the simple reason they where gone, but achievements earned where kept. Arrghh with that name, i never had a chance..