probably because
hex (0000000000000000000000000000000000000000000000000000000000000000) = dec(0)
hex (FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF) = dec (-1)
Interesting theory, but I think not.
Actually it seems to be a bug/feature in bitaddress.org as entering 0000000000000000000000000000000000000000000000000000000000000000 as the private key in the wallet details panel simply fails to update the previous result (I've been playing with it a bit more today ... it seems I must have entered FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF previously to 0000000000000000000000000000000000000000000000000000000000000000 and thus got the same result).
No matter then as all is now clear.
Its interesting to see what trivial keys have actually been used, for instance 1EHNa6Q4Jz2uvNExL497mE43ikXhwF6kZm has seen a few transactions (its empty now), can anyone hazard it's private key?