Why did reports on 99942 Apophis change approach dates?

You would think that with todays technologies calculating such things would be a piece of cake. We'll probably mine this rock for manufacturing, chew it up and spit it out as commodities long before it ever lands on earth of its own accord. Mining astroids, the next big of the creation. I say only because it isn't what you do that makes it , it's the purpose and state of mind behind what you do. But now I'm ranting. The thing I mean to say is this is a difficult calculation to make.