I've got this OF shot glass from the 50's. It's a proof selector jigger and can be used for measuring shots from different proofs. There are markings for 1, 1 1/2, 2 oz shots of 90, 100, 86 proof. What I don't understand is one oz of 86 is a touch less than 90, which is less then 100.

Why would the shot size go up when the proof increases? The opposite makes sense to me.

Thanks