Hi Wayne.
It makes sense to me, I was trying these interpolation modes yesterday. In the simple bounce (animator boot camp) I did three bounces with different interpolation to compare spacings.
* sine
* beizer + linear (or auto-clamped + vector)
*circular
When I reviewed the result, I perceived the sine version made the top turn around too quick, whereas the circular did it too slow. The one I created myself with beizer and linear looked best to me and it was somewhere in the middle of sine and circular.