This was hard to come up with a title for. For example, I've heard people say RE6 only got mediocre reviews because it wasn't survival horror, and people should rate it as a game first, not as part of a series. Same idea goes for Dead Space 3, Ninja Gaiden 3, Final Fantasy 13, etc. So should a game be rated on it's own, or as part of it's original genre/series?
If its part of a series, you would expect each game to be of equal or better performance than the last. It creates a certain expectation of the game, and you would hope it meets that expectation.
Of course one bad game in the series won't make me think poorly of the series, but it would be more disappointing to see a game go bad of its part of a series.
If its a standalone title without any previous entries, its easier to let mistakes slip past because you can chalk it up to devs playing with things to see how they go, rather than refining a proven system.