for the 1,000,000th time..
When you move up in tier... your avg goes through a "reset" and acts as if youre starting from 0..
So when you hit that +4.. your avg is now +4 since youve only played 1 round as a master when you play more rounds as master it will start avg'ing out between THOSE rounds.... pretty simple eh?
expect to see the same trend at each tier...