People that disliked something are more likely to vote on that episode to voice their dislike.

For example if the episode makes fun of christians you may have a heavy amount of christians vote the episode down even though it was a good episode. The people that thought it was good aren't as motivated to vote as the people offended.

This is also common with service providers and internet reviews. If the service is good there is no need to go online and post a review. If it's bad you are much more likely to take the time.