Unwanted z-depth during motion tween between keyframes

Hey there,
I’ve ran into this problem a few times.
I’ve notice that between two keyframes during a motion tween, a peg’s Z position will go from 0 at the first keyframe to 0.0003 B during the motion tween and back to 0 at the second keyframe. Why does this happen?
My peg’s position properties is set to Separate.
I’ve checked up and down the chain and it happens to a few pegs in this section of my animation.
Thanks for the help!