That ISO fits a generic guide set up by the ANSI standard committee on photography. It does not take into account all of the real world conditions. So, a tad of adjusting is going to be needed in extremes of sunlight and etc. Slight overexposure is one way to correct that without a lot of tests.

And yeah, we do know what we are doing. I was one of those guys doing it. I still use 320 for a 400 film!