I have noticed differences between receivers so far as receiving weaker signals, even though, as you say, sometimes they just give a different reading but perform the same.
I recently tried to receive the PBS channels on AMC3, and they are quite weak (at least where I live...just north of the border above Idaho). My Coolsat 6000 would sometimes show the breakup (square boxes, picture freeze), whereas on a newer VisionSat IV200 the same signal would show a good picture without any breakup. At other sites, I noticed that the Coolsat was better at picking up weaker channels than some older receivers, such as the SatCruiser.
So it's not just a matter of different reading...some boxes actually seem to be more efficient at dealing with weaker signals.