If you click the slightly hidden "get more feedback" button it shows:

Subject well framed: 100% Super sharp subject: 71% Great composition: 68% Pleasant blur: 57% Great lightning: 35% Well chosen background: 33% Interesting/original subject: 32% Good timing: 26%

So it's not very happy with half the categories but scores 9.4/10, so the score is definitely not an unweighted mean! (geometric or arithmetic).