Looking at the arctic fox's def and then dev moding in an artic fox to butcher one, I noticed it doesn't get a insulation bonus.
I believe it was intended to have one and was just a simple oversight.
Timber wolfskin - 100% insulation
Arctic wolfskin - 125% insulation
Red foxskin - 100% insulation
Fennec foxskin - 100% insulation
Arctic foxskin - 100% insulation
Edit: polarbearskin seems to have the same issue.