Some people are mashers, they push out at lower cadences on higher gears. They tend to have bigger legs (Grog pointed out Ullrich's cadence... look at his size too!)
Some are spinners, they have high cadence and use lower gearing.
It definately is partly genetic, the way you are built will dictate how much you can change this. But being able to do both at times is very beneficial.
It goes to the standard statement though, "Train your weakness, race your strength."