It is possible that mobile players (and now everyone, because of the new app) are promoted based on coin room wins and not level progression.
I was recently promoted to TM immediately following a win in one of the coin rooms. I have tons of ranked rounds and my average was higher than 63.
I have been winning many of my coin room matches every day since January. Since the promotion happened after a coin room win, it is feasible that my theory is correct.