The gauge is made to SAAMI specs (when 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 long. Not fitting the gauge tells you that you need to inspect these rounds more closely.
By looking at the cartridge through the slot you can see where the problem is, by seeing what is making contact in the gauge. It could be a problem with headspace, or body diameters, brass trim length, or bullet seating. All of these could cause the ammo to stick up out of a standard gauge and you couldn’t tell what was causing it. But the slot allows you to quickly ID which of those problems you have.
Note that the trim length for shouldered cases is not displayed.
