Well since those trolls aren't covered by the UOP, and it looks like an oversight on Bethesda's part, I'd have to say Kivan simply missed it. I can't come up with a logical reason for why only the easy troll has no troll fat.
It's too bad Bethesda chose to use leveled lists for death items instead of a normal inventory block. Things like troll fat make more sense as death items than as live inventory.
Easy Kalperklan Trolll is the same has well. no troll fat while other do..same way..
I can only think because these are leveled types of troll that the easy one did not get it because of troll fat properties once again..Like Showler stated..
Most likely Kivan missed them or nobody ever pointed it out..
I agree would much rather have creature rewards has death drop, but, you know bethesda did not think of it like that..or deathdrop is buggy..