Mind you some would say I only squeaked by my engineering physic classes
, but I THINK this is how it works. All objects ACCELERATE toward the earth at the same constant rate. Objects do not DROP at a constant rate, but rather ACCELERATE in a perfectly downward direction at a constant rate through the time of fall/flight. The longer the arrow is in flight the faster it is going toward the earth. The resultant velocity/force however(the sum of both horizontal and vertical aspects of the velocity/force respectively) will be less than the original velocity as well as determine it's flight path. This of course is within reasonable velocities.
A higher FOC is generally associated with a heavier arrow so it moves slower and therefore stays in flight longer prior to reaching the target. This "time of flight" affects the time in which the downward acceleration acts on the arrow. The longer an arrow is in flight the "faster" it is dropping by virtue of its downward acceleration. This flight of the arrow therefore creates velocity/force vectors, both horizontal and vertical components which are added together to produce a magnitude and direction resulting from both components.
The higher the downward velocity vector AND the lower the horizontal velocity vectors are (more time in flight), the more the resultant force/velocity points downward giving the perception that the arrow's point is dropping faster than the back. It is, however, not due to the FOC, but more so the resultant velocity vectors.
Hope this makes sense.