I know almost immediately an under-inflated ball. Makes a huge difference depending upon individual palm-hand-finger size measurements. Receiving also, depending upon the gloved receiver, hand receiver, finger-glove, bare hand,
and the body compression receiver.
Even more, not mentioned yet, is that the conditions were wet and rainy. Handle a regulation (league), ball, a wet ball, in different conditions, especially a fully (hard) compression ball vs. a softer supple % inflated ball, taylored into hand size. spiral technique favoring included.
“I know almost immediately an under-inflated ball.”
I guess you must find it odd then, given your “facts” that Brady and the Patriots performed far better with the balls in the second half than with the so-called “custom” balls in the first?