Not arguing that. But you still need to measure both weight and pressure. If the ball weighs more than 15 oz, you can account for that with moisture, and a heavier ball puts the user at a disadvantage. But if the ball weighs less than 14 oz, then there is an issue, rain or no rain.
So the same ball with the same air pressure would weigh different in Denver than it does in New Orleans. That’s why there is a weight variance given etc. The weight variance is not a 1 to 1 with the air pressure.