Contact gaging measures the dimension from surface roughness peaks of the part being measured (i.e. at plane A - A).
Open nozzle air gaging measures the mean surface of the part, which is approximately the average of surface finish peaks and valleys (i.e. at plane B - B). Technically, the mean surface would be an imaginary plane established by using the material from the peaks to fill the valleys until a level or zero line is formed.
The result is that there is a difference in measurement between air gaging and contact gaging. The amount of this difference is a function of surface roughness. The inside diameter of a hole will read larger with an air gage than it will if measure with a contact gage. Conversely, the outside diameter of a shaft will read slightly smaller with an air gage.
The following chart shows the diametral difference between air gaging and contact gaging.
Air Gaging vs. Contact Gaging
A New Dimension In Precision Measurement