kgermino is asking about the difference between (1) the actual speed (relative to the Earth) at the altitude where the plane is flying and (2) the speed of the path of the airplane when it is projected onto the ground. These are extremely close but not the same (~0.15%) because the Earth is a sphere and the airplane is at a slight further distance from the center of that sphere than it's projected point on the ground.

You are explaining the difference between airspeed and ground speed, which can be much more substantial (e.g., ~10%, or ~%100 in extraordinary situations).