If the troll you're talking about is the Seeker-hating Troll that Merrill and company die at the hands of, it may be due to damage it sustained in the fight with the Seekers. The battle doesn't seem completely scripted, resulting in actual actions being randomized every time the game generates the battle. In my experiences, even seemingly small variations in damage could have larger effects than you'd think, so maybe its an exponential scale of EXP gain vs. damage.
I can't say I've ever seen erratic EXP gains from the same exact enemy, though. Different groups have given different amounts (like the Orobas Fjords goblins in another topic), but not enemies in Broken Valley, unless Charm or NPCs were introduced.
