This is because the scale is far different to a 15 point or hexa scale.
In an ideal world. A 5 star rating should be a statement that says - This albums is in the same class as Sgt. Pepper or in Bowie case - The same class as Heroes, Ziggy Stardust etc
Rating of new release aren't judge by the standard of the history of rock/pop music. It's judge by the standard of contemporaries release. A high score by a professional reviewer only signifies it is one of the great album of that year but doesn't signify it is one of the greatest album of all time.
John Mc Hex D is essentially saying it is one of the great albums of all time. A hall of fame worthy album and a peak of David Bowie career
You can not know whether other reviewers think that highly of an album purely by metacritic score only by reading the review it self due to the vastly different scale and standard
Really a typical 5/5 score by a contemporary review is about equivalent to 11 to 12/15 or Hex A to Hex C.
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