From Dark and Darker Wiki
15% drop rate means the chances of not getting one in 10 kills is (1-.15)^10 ~= 20%. I'd say that's unlucky, but certainly not a statistical miracle.
Also note that Luck actually *lowers* the drop chance, so if you're farming for just Pelt specifically, unequip your Luck for the kill or simply don't bring Luck in with you. For some sample Luck->Pelt drop rate values, see the Quest_Cavetroll_Common tab at https://the darkanddarker.wiki.spellsandguns.com/Cave_Troll#Loot_Tables