I would guess they are really +/- 50% for Progressive because credit really does play a large part. I can't imagine they won't get sued again by these companies for falsely posting their rates as higher, so that they can sell more policies with the companies that actually pay them.

I would assume that they are testing their accuracy only via these insurers websites, not real policies. Going through geico's site, it doesn't seem to need any credit/real identification, so they probably aren't running a credit model (at least to get to an estimated quote).