Service velocity is ultimately about racquet head speed. Put differntly, Venus can swing her arm faster than anyone else. However, her lack of musculature works against her. Her control at that speed isn't so good. So while she CAN serve 127 mph, we don't see that go in much.
Serena and Lindsay can't quite match Venus for raw velocity, but as much more muscular people, they have a lot more muscles to control serve with. (Of course, that muscle has to be trained, which both these players have done.)
Still about once a tournament Venus serves the ball, and the oppoent sees nothing. They just hear the ball hit the wall behind them. Useful psychologically, if nothing else.