If we look at the Lord of the Rings trilogy, the films, Peter Jackson & Co finished shooting all the stuff in Kiwiland yonks ago. Sometime in 2001 IIRC. The reason for the seeming "delays" in between the three films (besides wanting to release them around Christmas times to guarantee fat sales) is because he has to do a shitload of CGI.
But you're missing one piece of vital information. The LoTR trilogy is dealing with artists and content, not code developed from scratch. They have the experience to make a realistic schedule, and they know what they're running into. Their bugs aren't crashing bugs or needing to work with specific hardware, their bugs are "oh, maybe we can make that texture look a little bit better."
Also, anyone in software development will tell you that release dates are frickin' hard to predict. The problem is that there are two many unknowns. You want to include all of these new DirectX9 features, but you get stuck because your video card drivers crash when you implement them. So, you're screwed until you get driver revisions from the GPU chip maker.
Software and new technology has unknowns. Movies don't. That's the difference.