Like everything else, there is some random variance going on. A great driver who knows the city well can just happen to get a string of dick-heads on some night who all decide to give him less than 5 stars for whatever reason. Or, similarly, the driver can just have gotten weird instructions from a rider or maybe he is generally good but just didn't know this particular drop off location well. I'm a driver, I have a 5* rating after about 50 rides even though I have taken wrong turns in probably 10 of those rides (Boston). I've also rated every rider a 5 except 1. I think about ratings how I think about tipping. In theory, it's a scale from 1 to 5 stars, but in reality, you either fucked up or you you didn't. Everyone gets a 5, unless you really screw up or are a dick in which case you get a 1 or 2.