Not only can it happen. It can happen when the GM is not the commish or even in any sort of official role in the league.
Check out this guy.
San Diego Team Index - HFTC History
Yeah he is in my league so I am biased and yes sometimes his methods rub people wrong (and make commishes life a nightmare, ain't easy being commish!), but you know his fans ain't complaining.
Bear in mind this is a 30/32 (expanded in 2010) team league that rarely has openings and never had a full season with a vacant team. So this guy is competing against 30+ other folks no AI for almost 6 real life years and puts up those numbers... demands respect.
There was an old thread where someone was taking nominations from commissioners for best GMs to start a league of only these nominated GMs. I nominated Eric for this league, but I don't think it got off the groud. I wish it did, I bet he would do well in any league.