I've noticed the same thing, and it surprised me. The first time I tried it, my entire body, neck to calves, ached for days.
Recently, though, I've been doing sets of 8 to 10 (just this side of failure!) and while it takes my back a few days to recover, my legs bounce back right-quick.
I assume my legs are just stronger than my back, which wouldn't surprise me. You might have the same 'problem'.
It should really only be providing secondary stimulation to the legs, look how far your bending your knees, not even close to parallel, and squatting shallow like that wont work you legs either