I replied to something you said, that's what it's getting at.
If you think you can only get 1 rep out, then do 2, how is your mind stronger up until that point? It's thinking only 1 is possible until you tell yourself, no, 2, my body can do 2.
(compared to someone who thinks they can do 1 more, which means they could only do 1 more)
IMO its harder to stop short to achieve your goal than it is to push to failure.