There's a combination of issues there. We do know the temperature outside and have a pretty good idea what the temperature inside was. We know that as the balls would have reached equilibrium, the pressure would have increased (going from a cold to warm atmosphere). We also know that when checking a ball, the act of checking it reduces the pressure slightly by incidentally releasing some of the air - I don't remember the amount, but its mentioned in the Exponent report.
There are also at least three major gripes with the Exponent's report that I've seen. 1. They didn't take some variables into consideration (evaporative cooling being one of them). 2. Assumptions are made to fit a conclusion. 3. The claim in the Well's report that is allegedly supported by the Exponent report that there are no combination of circumstances that would allow for the psi levels to naturally occurs is untrue, or at least is not factual - from what I've read they didn't satisfy anyone who should have been satisfied with their diligence.
I point back to my previous concern about exponents experiment, specifically their wet football tests. Rainwater is not ambient temperature, its much cooler. Spritzing a football with ambient temperature water and wiping it off immediately is not the same or close to game time conditions. Whether we can agree on what experiment is closer or not is one thing, but surely you can't believe it is close enough. Further, none of the balls were wet for an extended period (several minutes), while I agree some of the balls would have been wiped down, the efficacy of that is questionable and the idea that all the balls were remotely close is foolish I believe.
I believe the lower bound exponent used was too high, the balls would have been cooler and more wet. I think there are a lot of critics way more qualified than myself out there who are saying the same things. There are very few people to my knowledge defending Exponent's report.