Subject: Why would a cat have 6 claws on each forepaw but 5 toes on each forepaw?

I put this question in the development area but it could it easily be zoology, 
anatomy, or genetics. I think this question is allowable because the the cat is fine 
as long as his claws are trimmed. The cat in question is not purebred, a spayed 
male and very heavy. Before he was spayed he was fighting,as he was a stray. The 
strange thing about him is the first digit of his left forepaw has two claws on it, and 
the first digit of his right forepaw has two claws on it. The other digits on his 
forepaws are normal. He has four normally clawed toes on his each hind paw. All 
toes are big, but he's a big cat. The third phalanx on a cats toe is tiny, so maybe 
his first digit branches after phalanx 2 although this seems unlikely to me, for the 
pair or claws are both proportionially sized to the claws of digits 2 through 5. I 
thought it might be two entire digits in one sheath of skin. I discounted that idea 
since the first toe is not wider than it should be,and he can retract both the sets of 
double claws half way into what I think are their own sheaths. His large size and 
easy going manner make me think of trisomy 21,but his history of fighting makes 
me think it can't be that. Please tell me why a cat would have 6 claws on each 
forepaw but 5 toes one each fore paw. Thank you.

Re: Why would a cat have 6 claws on each forepaw but 5 toes on each forepaw?