The gauge is made to official released cartridge/chamber specs (SAAMI/CIP where applicable). This includes both length measurements like the OAL and headspace, as well as the case diameters. If the brass does not fit all the way in, it could be because one of the length measurements is off, or it could also be because of a diameter is too large. The gauge tells you that you need to inspect these rounds more closely.
Looking at the steps on the face of the gauge, they indicate the chamber min and max length (as shown on SAAMI drawings), and the spec min cartridge headspace length. By sizing the cartridge to be at/below the chamber min(middle step), that ensures that the ammo should fit any chamber in that caliber. And by sizing it to be longer than the cartridge min length(shortest step) that means that it will be be a good fit, and it shouldn’t have any issues related to sizing.
Note that the trim length for shouldered cases is not displayed.
