Help me with wacky keyframes madness once and for all

I've been using Premiere for years and years, I know almost all of its nooks & crannies. But this is driving me nuts.

Steps to reproduce:

1. Throw a clip (anything over 10 sec. long) on your timeline
2. Add 3 Position keyframes, roughly in the beginning, middle and end. Confirm the values for all are the default 960,540.
3. Change the value for the 1st keyframe to 960,0
4. Change the value for the 2nd keyframe to 950,540

Play and behold...  between the 2nd and 3rd keyframe, the clip moves in the Y axis! 

And yet the 2nd & 3rd keyframes are from 950,540 to 960,540. So instead of a slow movement to the right, it's adding a down&up movement (my Y value peaks at 579.9 before returning to 540).

- This has been in Premiere forever, and I just tested the same behavior in CC2018 and the latest CC2020. (OS X 10.13.6)

- If you try to force a "fix", e.g. adding a 955,540 keyframe inbetween, Premiere will defy you, doing the same up&down maneuver twice now... so you can slice&dice it ad infinitum.

- I used to think it had to do with bezier algorithms I didn't understand, but as you can see, it has nothing to do with that and the behavior occurs with linear "dumb" keyframes.