It's a quirk with the polydactyl gene. Sometimes they get an extra toe instead of the 5 they are supposed to have.
Usually this is just a little bit extra splitting off the back toe. I have had full on extra toes in there.
Shouldn't breed them as you don't want that trait in the lines. I've been fighting to remove the double toenails I have pop up evey now and then which is a related issue