When you get promoted to Legend or any Tier, the first round you have in that Tier is your Ave.
Of course there is a few other rules relating to that. There is quite a few TM's with Ave's under 60 at the moment, but when they play the required amount of stroke ranked rounds, then their Ave will change or maybe not change.
I remember (awhile back, don't know how I remember..lol) my 1st round as Legend... my Ave went to 70 or something like that and now it's slowly going downhill.
-Roger