clearly i'm missing something
hehe yes, you do indeed.
Look at your frames again. This time look closely to the bottom of your sprite.
On few frames there's a gap between sprite (red pixels) and Hotspot (or better say bottom of your frame).
You got Platform Behavior on that sprite (to be honest I never used it) but as far as i know it detects collisions betwen "pixels of your sprite character" and "pixels of the ground sprite"
And now you have set Hotspot to bottom for every frame - that's fine, but betwen hotspot position (very bottom) and your most bottom pixels there is small (usualy pixel or two) gap.
And now (again) when you move, your sprite collides with the ground but it colides betwen pixels of sprite and pixels of ground, not betwen hotspot of sprite and ground.
Look again on my cap file. I fixed that problem by moving almost every frame of your sprite down. So the bottom pixels Y position is equal to the hotspot Y position.
or maybe you want me to make a picture with that problem area? and show you exactly whats your problem ?