The gauges are made to the minimum chamber dimensions. Most chambers will be larger than this. So it will reject some rounds that may still fit your chamber. If you only use the gauge as a pass/fail chamber checker, any rounds it passes should fit into your chamber without an issue.
But using the gauge just as a pass/fail chamber checker is missing its use as an inspection tool. The slot makes it easier to pinpoint interference, and the 3 steps on the face are directly pulled from the headspace reference specs for both the cartridge and the chamber.
Since the dies and other parts in the reloading process are all designed around making ammunition that will fit an in spec chamber, then the question should be why wouldn’t it fit the gauge? Even if it does still fit your chamber, the fact that it is failing the gauge is telling you there is something in the loading process that needs to be looked at. It could be a problem with neck thickness or concentricity on brass formed from other calibers. It could be neck bulges from the bullet seating process. It could be that the bullet is not seated straight. It could be burs on the case mouth from cutting the case. There are a long list of other potential problems that can be found using the gauge as an inspection tool.
Some people just want it to fit the one firing chamber they test it in and aren’t concerned about any quality control beyond that.