For user score, it ranks whichever has the higher percent of users that “like” the album, which is the percent of ratings that are 70 or higher. For example, Radiohead’s OK Computer is ranked number one on the site’s highest rated albums of all time, even though it has an equal user score and less ratings than Kendrick Lamar’s To Pimp a Butterfly, because 98% of users that rated OK Computer gave it a 70 or higher, while only 96% of users who rated To Pimp a Butterfly gave it a 70 or higher.
For critic score, I haven’t got a clue. The amount of critic reviews seems to be one of the parameters of tiebreaking; the site lists the two perfect-scoring albums with six reviews, Prince’s Sign O’ The Times and The Beatles’ Revolver, above the seven albums with five reviews, the 13 albums with four reviews, and so forth and so forth. However, I have no clue as to why Sign O’ The Times is number one and Revolver is number two…