My guess is user over-ride to skip that particular test? CB seems to auto-skip if % falls lower than already-best result set, no?
It seems this does, indeed, skip through the current test.
What it actually seems to do is lower the number of captchas CB will attempt before starting with the next test.
So, if you hit 'Skip" while CB is on the third captcha in the list of 100, CB will do all of the current tests on a certain parameter up to the third one, and then move along.
I guess this is useful when you see it going through total junk settings and you don't want to waste time/energy.