When you kill a type creature a lot, you become a specialist killer in their kind. Firstly, you gain bonuses in killing them as you know their weaknesses, hence the quicker killing. Secondly, sometimes your character gets adventurous and tries a new combat trick when killing a monster (eg. "Let's see if I can kill it with the flat side of my axe."). This self-penalised combat provides the experience bonus. There is no actual penalty for this special move because you are already an expert at killing them.
None of this is shown visually, but the bonuses and experience are still very welcome. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" /> Enjoy!
