Also, I can't prove this to be true but...
Delta seems to open seats (except at the window) 1 at a time. If traveling with a companion and we don't get upgraded at the window I find that my only chance is at the gate.. When 1 seat opens I think you are skipped to the next medallion. U only get upgraded if they actually open 2 seats...
My wife is gold so at times when only a small number of seats are available I have opted to split the PNR and at least get 1 UG to clear. Especially on medallion heavy routes.
Perhaps my observation is wrong....