Note: Thirteen is significant in this context because thirteen is one more than the number of Fobott'r fingers and thumbs. In Fobott'r numerology 13 means "spare" or "redundant," which fact makes Peri's leap of logic look more like a simple step (which, in Fobott'r numerology, would be represented by the number 2, because their word for "2" is one of their words for "feet.")

Addendum to Note: Because you'll ask: "Finger, feet, hand, arms, head, self, crest, thing, strike, secret, magic, power, spare." Now you can count to 13 in base 12 like a Fobott'r numerologist. Not that you should need to. Seriously, this time you can trust me.