The thinking goes like this.
We have a lot of forums since we cover more sports than just NBA or NFL or Football. So when spammers come along and do their thing, we want guys to be able to edit. Those would be the GMs. Ideally, we'd have moderators for every forum and they could do the job, but we're not big enough yet.
On other sites, the GM type role would do what our producers do. That is, manage a smallish section of the site (e.g. NBA Atlantic, NFC North) and work with the mods to make the place fun.
Since we have more than one sport, the commissioner role makes sense - if we broke up the site into separate sites per sport, those guys would be the admins of those sites. That is, NFL commissioner would be Admin of our football site.
Make sense?