I have noticed this bug in the addons store. Any addon with just 5 star reviews (does not matter if it is just one or more, all of them 5 stars) has its listing showing the addon only with 4 stars.
An example of this problem can be observed on this addon: https://addons.opera.com/extensions/details/pdfaid/
Notice how it has only two reviews and both are exactly 5 stars? Now look at the top and you see the addon with only 4 stars, despite the fact that the only existing reviews are all 5 stars.
This problem is also present in the listing of addons, meaning that there is absolutely no addon for opera with 5 stars just because of this bug since it is limiting the max review to 4 stars, for some strange reason.