Bug in editing Motion Tweens?

I’m brand new to Flash (started last week). Playing with motion tween editing and thought I understood the difference between using the selection versus sub-selection tools.

BUT… Under certain circumstances (and I haven’t been able to get a “handle” on what those circumstances might be), I seem to get a control point on EVERY frame, whether a key-frame exists at that point or NOT on the TL (also, whether I got the sel or sub-sel tool selected makes no difference).

I’ve searched the docs and found no mention of this behavior. Anyone know what’s happening here???