The pattern seems to be an effective floor of 7.4 until midway through Season 8, a floor of 7.3 until Season 11, and a ceiling of 7.3 from then on.

I can't think of why that would be the case, or how it makes sense.

I agree, no other TV show on this site has this kind of pattern. And the oddest thing is the ratings of season 1. Because, really it was awful.

The distribution is a result of the low number of ratings. Look at the number of ratings for each episode. For all those early 7.3-7.4 episodes it's about 600 ratings, the higher rated episodes have more ratings. Then from about season 12 there's a drop in number of ratings to about 300-400 for most episodes.

Compare this to the Game of Thrones ratings[1], where each episode has about 3000 ratings.

[1] http://graphtv.kevinformatics.com/tt0944947

It's pretty clear that Simpsons fans hate Lady Gaga and Musicals.

Really, the Lady Gaga episode was awful. And not only because it was a very obvious 22 minute advertisement. It was just plain bad, very bad. I absolutely agree that it's the worst Simpsons episode I've watched.

The individual ratings may not be very "accurate" (well, they're going to be subjective no matter what. My favourite episode is near the bottom of its season), but the trends are interesting. The fact that the floor of seasons 1-9 is above the ceiling of season 14 and onwards (save a few outliers) reflects quite well what many fans think.