If you trawl around the Net you will find (somewhere) the formula that is used to calculate the average rating.
Sorry - I just can't remember where - any ideas James if you're there?
It has alot to do with the actual amount of traffic you work as well as safety, obviously. For instance if you did a quiet session you would get a much lower rating than doing the same things on a busier session. Hmmm that sounds very obvious
I'm a 96 man myself