The 5 star rating system being used reminds me a lot of eBay's feedback system. eBay's system has changed quite a lot over the years, and much could be said about each change they made, but specifically the 5 star system and the bar they set was always problematic because of the disconnect between what buyers thought a fair rating was vs what eBay considered fair.

In eBay land, anything less than 5 stars is unacceptable, but for most normal people if you asked them they might say 3 stars is fair for a perfectly acceptable experience. If you get too many 3 star ratings from happy satisfied customers your average might drop from 4.9 to 4.7 and suddenly you're on thin ice and at risk of having your account closed.

Looking at this article it appears Uber is using a similar system. They require an average rating of 4.6-- meaning every 4 star review you get is actually a ding against you-- even though those 4 star reviews might've been perfectly happy riders who just felt in their minds that 4 = Good, 5 = Outstanding or something.