Because the velocity was not constant. The spacecraft was accelerating during its cruise phase to Mars. The velocity relative to Earth at launch was 24,600mph. When it approached Mars, its velocity relative to Earth was 64,123mph.

That's a roughly 8.193mph^2 rate of acceleration over that duration on a curved trajectory. You also have to consider the relative speed of Mars' orbit...

So the distance over time does add up, but you need to consider the movements of other celestial bodies and use some more advanced math to get there,