Two reasons:

1. Ground speed is not the same as air speed. So a plane flying at 500 knots into an 80 knot headwind will have a ground speed of 420 knots.

2. Fuel efficiency. In the same way that flooring the pedal in a car burns a lot more fuel than driving a little slower, the same can be true for planes. So they may run the engines at less than 100% power to burn less fuel. There is probably some calculation they do to get the optimum engine power and speed.