Go to the part where Gustoff has just started hunting Pest and Pest falls down a hill. When the snake starts crawling on him, his legs are as bald as a baby. In the next shot, they're hairy.
I think they only LOOK smooth because the camera is a bit further away as well as less focused. When they zoom in on his legs when the snake is climbing up you can see clearer and you see all the hair (gross).