Bouncing implies you actually hit the ground, he would be more of a launcher
Well, it kinda looks like a bounce sometimes. The club comes down, smashing you into the ground, you ragdoll and bounce off of Nirn?
Or, if there's no other explanation...it's magic.
Or, the giants are actually a race of dragonborn who have discovered how to craft shouts into their weapons. They favor Unrelenting Force and laugh at the little humans they send flying.