Something I only learned recently. Props actually use two forces to move the plane. The first is newton's law - I push something out the back (air) and to conserve momentum the plane moves forward. The second force is the bernoulli affect on the prop - the prop is actually a little wing biting the air. Like all wings, it has an optimal angle of attack with an air stream at which it produces the most lift (and in this case lift drags the plane forward), or is most efficient. At different speeds the angle of attack against the propellers blade changes due to the component of velocity moving forwards. This means that by varying the angle of attack of the propeller you can vary the most efficient air speed for the propeller.

I think others have said similar, but my guess is that slow flyers are optimized for a lower airspeed.