These are called floating point errors. Unfortunate result of how computers do math pretty much.
I figured as much, it's pretty obvious it's some sort of rounding error. I think it may be a sub-bug in a bigger project-specific bug I'm combating, so I'm trying to find the reason why.
If the Width value of X frame in Animation Y on Sprite Z is 173 pixels, it should always be 173 pixels... If it was consistently rounded incorrectly, I could work with that (.1 + .2 = 0.30004)
The longer the animation plays, the more the number changes. The first playthrough actually has correct values. I'm filing a bug report, I was able to re-create in a .c3p with nothing but a single sprite.