You can’t tell much between an 11 PSI ball and 13 PSI. Maybe about 1mm inward deflection from the grip of the thumb squeezing it.
That's incorrect. Think of it another way. Those balls were 15% to 20% under inflated. I played some college ball as a center. the "cushyness" of the ball make a difference, especially when it's wet. In fact, in shotgun formations, I liked the harder footballs because the slid of the turf/grass easier and more consistently than the softer balls.
That said, college balls are much different than professional balls.
What I am getting from all of this hullabaloo and the NFL’s statement about this issue is that the refs probably did not check every ball with an air gauge before the game as they probably should do. I am guessing they checked one with a guage (the one that was properly inflated) and the rest by feel. That is the only reason I can think of for this not being an open and shut thing.