Have you ever rated something on NG unfairly low only because you were extremely upset, sad, depressed, frustrated et al. or generally in a bad mood at that very moment?
It has happened to me. Quite a few times I found myself revisiting movie or art submissions I had previously impulsively rated 0-1 stars due to being angry or sad et al., and thinking: "man, this is actually really decent" and then raising my initial ratings.