You do know you can
buy reviews on Google Play and a lot of companies do just that if you think the scores there are completely trustworthy you are very naive ( I don't believe Mantano or Moon does this by the way but it happens), not to mention the reviews are split between different versions of the app and not just the current incarnation theres also the matter of Moon+ being available for a lot longer before there was real competition, the difference in pricing means people expect more.
The reasons for scores are so varied the only way your going to get any idea is reading all of them.