Actually there is no bug there at all that I can see. If someone places a comment on a photo and does not rate giving it essentially a 0 rating should it not be averaged? That is how all those things are done its all averages.
The issue I see it is instead the calc_rating function in pp-inc.php has this line and only recalculates ratings based on if they are no 0 ratings so essentially thats where the bug is. You say have a photo and it has 2 10 ratings and 3 people who commented and gave it a 0 rating. Your still showing the photo rated as 10 but to me looking at the math the real rating is 2. Thats why it no longer shows in your highest rated block because its average is no longer than high.
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