I get the sense that Stevens decides who plays and who doesn't (or who continues playing during a game) based on different factors than we, as fans, are used to looking for. We see a relatively close game and think, "Why the heck is Green on the bench?! He's their best scorer!"
Meanwhile, Stevens is probably focusing on things like effort and the cohesiveness of the players on the floor as a unit.
As for Humphries, I assume that there's a reason for that as well. There was talk about him being a hard worker this summer, but if Humph is simply not in the rotation despite his talent and experience, I have to conclude that he's not working that hard or doesn't seem to be buying in and working well with teammates.
All speculation, but it seems at least as likely to me as the "blatant tanking" theory.