I have been asked to explain how the Super League Averages are worked out.
  It is how you do them on score sheet. Legs x 501. minus score left, divided by the arrows used.
So if you have played say 10 matches. I do not add your averages together and divide by 10.
It is the number of legs x 501, minus score left, divided by arrows used. this gives a true average.
By just adding the averages together you can end up being several points out.
Hope this helps