Over the course of my research into the mysterious AniRec Database, I've discovered an interesting phenomenon. To be fair, this is probably already blindingly obvious to everyone... but I've given it a name!
Sequel Bump (n.) - The tendency for sequels to have a slightly higher average score than their original. The effect tends to add as little as 1/20th of a star, all the way up to an entire star to the score of the sequel. This is assumedly because people who did not like the original will not watch the sequel, so the series has a built-in audience of people who are primed to like it.
The moral? Be wary of the ratings of sequels.