Or the referee room where the air pressure is checked has low air pressure. That 12.5 to 13.5 psi is a relative air pressure. The air pressure in the ball is 12.5 to 13.5 psi greater than the external air pressure.Maybe it was an honest mistake - like checking the pressure while they were in a 200 degree oven.
Probably most people have worked in a buidling where the doors won't close when the weather's just right or the doors are incredibly hard to open when the weather's just right. And it's usually the same doors that have a problem. It's because the air pressure in that room or that part of the buidling is different than the rest of the building.
If it's hard to open the door to get out of the referee room, then your mystery is solved.
Not to mention that the difference in indoor temperature and outdoor temperature would have an effect. Except, unlike a few articles have posted, you have to use the absolute pressure inside the ball, not the relative pressure, when using the PV=NRT equation. Atmospheric pressure is usually around 14.7 psi, so they should be plugging approximately 27.2 psi (and using Kelvin for temperature) when figuring out how much pressure is lost due to the temperature difference.