I recently asked the same question about my Freedom and received the same response.  There's no increase in AF between the regular Freedom and the Signature.  My theory (and, being one of RyVision's hated lawyers) is that the introductory interest rate of 0% may need to be canceled for the PC, which would constitute a rate-jack under Section 172.  Whether that's the case or not, I have no idea.  It's the only justification I can fathom.