According to Wells report:
"
the Ideal Gas
Law predicts that the Patriots balls should have measured between
11.52
and
11.32
psi at the
end
of the first half, just before they were brought back into the Officials Locker Room."
If you take Prioleau's numbers alone, only 3/10 are below their own stated expectation in drop. Their scientific investigation seemed only to concentrate on temperature and very little (if at all) on effects of cold rain on PSI.
Why only use the higher number? That's the justification why Colts balls were in spec, that their footballs were at least 12.5 on one of the gauges.