A source very close to the Exponent research team has informed me that Exponent used a hand-held spray bottle to simulate the heavy rains that were falling at Gillette Stadium on January 18.
The researchers would squirt the football with the spray bottle, then immediately towel it dry. They then waited 15 minutes and repeated the procedure again.
Because the "on-field" portion of the simulation lasted about two hours, this would mean that the balls were only sprayed and dried a total of nine times.
Exponent claims that this spraying/drying at 15 minute intervals somehow is the same thing as the sheets of cold rain that poured from the January skies in New England.
Even this light amount of moisture caused the footballs to lose a significant amount of pressure, and it's entirely likely that a higher degree of moisture would cause the balls to lose even more pressure. We cannot say for sure, because Exponent did not test varying degrees of moisture; they only tested the light spraying and drying described above.
For all we know, heavier levels of moisture would have caused the PSI in the test footballs to drop even further, which would have given us a more accurate idea of how much pressure the footballs should have lost that day.
Remember, the Ideal Gas Law assumes DRY footballs. The widely-publicized expected pressure range of 11.32 - 11.52 does not take moisture into account in any way, and would be significantly lower if it did.