Jan 9th 2010 9:09PM I think one of the reasons why Wrath of the Lich King took so long to release was the fact that balancing out Death Knights (as well as other classes) sapped so much time and energy from "real development" of the game. I was in Wrath of the Lich King Beta and it seemed almost every new build that came out for it was a bunch of fixes for Death Knights. Unfortunately, this may be the case for other expansions introducing Hero Classes in them.
But, that may not be the case for Cataclysm.
Also, stuff happens. I could hire workers to build 2 identical buildings in different parts of the world, but one may finish faster than the other due to factors like good weather or stronger labor. My point is, you can't really compare previous expansion timelines with current ones.
Though they do make for good speculation and predicting ;)