You got the right idea, but your all backwards.
The engine is sucking on the intake plenum like a guy sucking up a thickshake with a straw. The harder the guy sucks, the more vacuum is created.
(The more resistance on the intake side of the engine, the more vacuum is made.)
When the throttlebody is only cracked open a little and the car is cruising along, its not sucking in that much air, hence less vacuume will show because the guage will be getting closer and closer to 1 bar(omatric) unit of pressure, which is 14.7psi, depending on your altitude of course.