User blog comment:CoyoteDork/Season 6 New Series Regulars (Speculation)/@comment-5014364-20160115051427

The distinction between "Regular" and not is purely trivial, the result of contract negotiations and the like. Many, many major characters within the storyline were technically not credited as "Regulars" for long periods of time, and many still aren't.

I wouldn't waste time thinking on it.