How come boost gauges have numbers before zero?

A turbo-boost gauge has a vacuum half and a boost half, thereby being able to show either negative or positive pressure.

i knew a guy one that had the K&N typhoon CAI on his accord, and installed a boost gauge(with no negative numbers) and a vacum gauge and said he was getting like 3 PSI. i was just wandering even if he was why would he get a vacume gauge?