I think the issue regarding Dedich/Limmer is this:

Dedich is a better OG than Limmer.
But, Limmer is an experienced Center who can play both OG spots. He is a better Center than OG.

So, it would make sense that if one of our starting OGs was a game day scratch, then you start Dedich. And then make Limmer the reserve for OG and C.
Its a similar reason for dressing Quesenberry and not Humphries. Humphries is purely OT. Quesenberry is OT who can play OG. So, if you have a starter OG out, your other starting OG gimpy, then you suit up Quesenberry.