A possessed is different from a mutant, you have to be clear about what your refering too.
A Possessed has a base of 2 attacks at str 4, which means if you add a crab claw you get one more attack at +1 str. Also if you add in the scorpion tail you get one more attack at str 5, that is a total of 4 attacks.
A mutant with a crab claw will make one attack at str 3 if your holding a weapon in that hand, your second arm becomes the claw giving you one more attack at str +1. This also means that hand cannot hold anything else because its now a giant claw.
These example get confusing but if you give a mutant 2 crab claws, it just has two attacks each at +1 str. Its easier to explain in person, but they faq alot of the mutation stuff.