The difference between the two, and thus justifying a higher rate is, garment tanneries focus on the body of the hide. The main portion of the hide is the only thing a garment company is interested in.
Garment tanneries can process them at an unbelievable price and pace because their is little or no work what so ever done to the head, tails, and or paws.
Taxidermy tanning requires many more hours of work. The faces need to be thinned, the edges need to be cleaned, full body mounts will have the animals limbs attached which need to be fleshed, sometimes along with paws, ears, and obviously the tail. All adding up to much more attention during the process.