lakersout,
I've skinned Rugged leather off of a few mobs my level at 45. I couldn't say for certain, but I'm fairly confident that the type of leather you get when skinning is dependant of creature in which you've skinned.
When I've run around for fun helping friend(s) of considerably lower level. I've never recieved a leather better than when I was that level and skinning the same creatures.
Also when I need X leather for a recipy I've then started to "tag," certain location for what type of leather they offer when an animal is skinned. I.E. heavy leathers are dropped in Strangle horn Vale by raptors level 42 ish and tigers. But if I wanted say thick hides I'd have to make sure my prey was no lower than level 44 Lashtail raptors and such SHV.
If you like try this experiment on your own, hack and slash a sheep and skin it. You'll find that your results are no better now than when you tried the same (and I know you did! I sure did.) when you first saw one.