When you get many more ratings/ raters the average rating will often go lower.  Here we often get very high ratings from a limited set, the fanboy type.  There many people may be rating many more albums and of a wider variety. 3.9 is still very good and some here have complained that we get far too many 5 star ratings, and that those should be very exceptional.  Of course some here hand out 5 star ratings and one star ratings like candy....

I for one only have liked to rate/ review my favourites, so it was bound to be skewed high and prefer to judge my rating on how much it appeals to me (personal value).

Ratings only work as a rough guide for me, and have been of most value to me when deciding on which albums of a discography to try first.

I think that ls how all music genres are perceived:  a  few outstanding ones, which seems definitionally true, and most (the average album) will be considered to be of average quality.  I wouldn't expect otherwise, but since people tend to choose to listen to kinds of music they like and bands that interest them it's expected that they will be more likely to rate on the higher side than how most would rate that album if they heard it.  If one has a very large collection and is comparing to a wide variety of music, then one may well be more critical (more to compare it to in terms of quality even if/ when much of that quality is merely a subjective perspective that illustrates one's biases).