WTA's ranking system keeps the best players on top. That's why there are few upsets.
Look at Pete Sampras. He hasn't played since the beginning of the year. He is now ranked in the 20s, 30s.
Compare that to Lindsay Davenport who, despite the fact that she missed 9 months of tennis last year, was still ranked in the top-10.
WTA has a very stable system. Just because you missed a tournament here and there or just because you lost early in 2 consecutive tournaments doesn't mean you no longer belong in the top-10.
ATP ranking is like a hot dog contest like someone said. whoever is the hotest dog at the moment leads the pack. You miss a couple of tournament and your ranking crumbles.
The guy who beat Hewitt at Wimbledon didn't even get any Quality Points. So in the ATP, it doesn't matter whether you beat the world #1 or the #300, you get the same amount of points. That sucks.
In the WTA, it's not easy to reach the top. You have to play consistent tennis for 52 weeks. And when you get there, it's not easy to drop to a low ranking. You have to lose consistently for 52 weeks.
I hope now you understand that WTA ranking system doesn't allowed wild swing, therefore, produces few upsets.
The best are on top, the rest is at the bottom.