We are presuming the referees did their job of inspecting the balls before the game. The refs have pressure gages and are given the balls 2 hours before the game.
According to a story by an ex-ball handler for the Chicago Bears that was posted here a couple of days ago...
He said the refs would not always take the pressure with a gauge, but rather feel them and if they felt “ok” would then take possession. They’d only take the pressure with the gauge if the balls felt especially soft or overly hard.
HIS speculation was that the refs knew Brady liked soft balls so figured that’s why they felt softer and let it be.