Tank Power Rating

I just wanted some clarification on how the power ratings of tanks is calculated nowadays. I keep entering battles hoping to clear up exactly how skillful certain power ratings are but they are all over the place and seem to have no real order to them. I go into legend battles where first and second are 8000 and 3000 consecutively and third place is at 3000. Others still sometimes even sit at 0. When I log in the number sometimes changes while i'm in the garage not even upgrading anything, just sitting idle in the chat.

If someone could clear how these ratings are decided that would be an incredible help.