Whoops, did my math wrong. 10.5 psi ball is 19% below average pressure of 13 psi ball.
Yeah, but the legal minimum is 12.5, so they were 16% below the level allowed. Sounds like it’s a pretty common thing for QBs to try to sneak the same past the refs, however, and more QBs are supposed to like their balls under-, rather than over-, inflated. Also, the balls lose pressure in the cold and this is January, so they’re probably used to balls at or below 12.5 before they’re taken out to the cold, so the difference may not have been that great.