A few points to bear in mind.
The +1 on its initiative obviously suggests a high Dexterity.
All crabs have 10 legs (the first 2 are claws, the rest usually for walking), but it says this chap has 4 legs.
The explanation is simple - it's a hermit crab.
Hermit crabs have claws on their first pair of legs, walk on their second and third pair, and the next two pairs of legs are very small and used either to hold the shell they hide in or clean their gills. So, only four of its legs are "legs" in the walking sense.
What's more, the World's largest living species of land arthropod is a hermit crab, the
Coconut Crab.
I suspect this "Giant Sand Crab" has similar habits, and probably has young that hide their soft bodies in stolen shells or coconuts, but the vermin's chitin grows thick and hard when it reaches a certain size.
Oh, and it has 2 claw attacks AND a bite, obviously.