Zangoose is based specifically on the Indian Grey Mongoose (as can be seen by its long, bushy tail and zigzag stripe fur pattern, as well as its rivalry with Seviper), which does not use its claws for any sort of scratching or slicing. Rather, all of an Indian Grey Mongoose's offensive power is in its jaws. Doesn't explain why Zangoose can learn all sorts of other claw-based moves like X-Scissor, Night Slash, Fury Swipes, Slash, Swords Dance, and Crush Claw though. Most likely it's that Zangoose's claws are not well-equipped to cut, but to pierce and scratch.