I noticed my ex-girlfriend rating our Uber driver 3/5 stars once we arrived at the airport. I asked her what was wrong with him. She said he was fine but it was just an average ride, nothing exceptional. And that she's only given out one 4/5 and zero 5/5's.

Not her fault. What choice do you have but to assign your own arbitrary meanings to each star, and how aren't those going to vary from person to person?

I never looked at 5-star rating systems the same.