There is probably a disappointment factor if a guy goes to get topped and then the girl is unable to perform which would lead to lower scores. Again, research is the best way to avoid disappointment in this hobby.
Also, getting fucked in the ass is pretty fucking intense. When it's good, it's GREAT! Not that fucking a girl in the ass isn't great too, and I have given plenty of high marks to bottoms. Perhaps the reviews you read were from guys who are new or don't get fucked often and it may have been an intense session. I am more of a top, but will bottom if I'm super turned on. So the sessions may have been going well already, the guy decided to bottom, and the session got even better.  
Honestly, it's probably best not to read too much into it. Both topping and bottoming are great if you click with the provider and are having fun together. Again, research will help with this...