I feel like someone is tampering with the ratings. I've seen this happen with episodes of seasons 9 - 12. The ratings for those episodes used to be higher earlier this year, but for some reason, they've dropped.
I've noticed a trend in most of the season 12 episodes: Their ratings start off rather high, then about five days to a week after they air, the ratings crash land. Last week's episode, "Boopa-Dee Bappa-Dee", had a 7.2 rating earlier this week, but as of November 22, it's at a mere 4.9. I think it is most likely for one of two reasons: either most people (for some reason) waited to cast their vote, or someone is tampering with the ratings to make them look bad.