Because they're not the exact same breeding.

In fact, the parents are different, with different parents themselves, and because the tightness of each parent is different, it thereby affects the aggregate quotient.

I understand what you're saying ... in the sense that all 4 grand parents are "the same" ... but because they're stacked differently, therefore they put a slightly different spin on things for each parent.

For example these two dogs have the exact same 4 grand parents also:

http://www.thepitbullbible.com/forum...hp?dog_id=7686

http://www.thepitbullbible.com/forum...hp?dog_id=6826

However, their W.I.C.s are different as well. Miagi is a bit looser-bred, while Serious is tighter-bred, and that is because Serious' daddy was a 75% inbred Poncho dog, while Miagi's mama was only 50% "double-granddaughter."

So they're really not "the same" at all; just very similar.

That's a good question though ...

Jack