What is the deal with the uncontrollable motion curves?

Shrug. I already solved my issue by doing something stupid and wasteful, but I'd still sure love to know... Just what is the deal with AE's uncontrollable curves?

I set up a keyframe for position A, set up another a few frames down the timeline for position B. I want the motion to be STABLE at position A for a while, then ease into a rapid acceleration and deceleration for position B, and then remain STABLE at position B for a while.

AE is having none of it. It'll give me my acceleration/deceleration curve, sure. But remaining stable before and after that curve? Forget it. Keyframes be darned: It's going to go all over the place.

First thing I tried was setting up keyframes on every single frame before and after the curve. You'd think the motion wouldn't budge then, right? Well, it does. It literally VIBRATES with the desire to do what AE wants, rather than what the user has gone through the trouble of specifying.

So then I tried eliminating the curve altogether and manually inserting the values it had returned, so that instead of a curve, I have the effective equivalent across the same span of frames. And what, oh, what does AE do? It treats it like a G D curve, complete with the apparent inability to hold steady before and after the motion.

This is rubbish. I "solved" the issue by duplicating my object several times, putting them at position A, position B, etc., and keyframing their opacities so that I did, finally, get the effect I should have gotten had AE's curves been, you know, controllable.

Maybe I missed the switch, buried somewhere deep in AE's perferences, that says "Turn off arbitrary motion inexactness". You know, kind of like how one has to turn off the edit softener in Audition before they can really start doing anything precise.