Miscellaneous: "a shark" drop rate of shark skin

[Lemonhead]

In Erudin, "a shark" doesn't seem to be dropping shark skins, or has a very low drop rate. The lower level "a sand shark" is dropping at a normal rate, but I've killed many "a sharks" with no skins while "a sand shark" is 1/3 or 1/4 drop rate.

I would assume the higher level shark should at least have the same drop rate of Shark Skin as the lower level sand shark.