I would go with what is said in Post #2 - and add - that Hornady brand tool is likely a made-in-China thing that you are relying on to be dead-nuts accurate - you are chasing numbers, when what you really want is your brass to fit to your chamber - does not really matter what that number is. If the tool is not helping you do that - and you are using it correctly, then most would call that tool "a piece of useless junk" - and I own and have owned many pieces of "useless junk".
Perhaps for clarity - for decades, maybe for centuries - "headspace" was measured with steel ground gauges - used to set the chamber in a barrel - usually a set of three gauges - GO is minimum length; NOGO is about 0.004" longer and Field is Maximum length - most barrel installers / chamber cutters will try to be between GO and NOGO - I can not find where SAAMI defines NOGO - it might be a barrel maker or reamer maker invention, to help speed up chamber installation. SAAMI defines Minimum - that is the GO gauge. And SAAMI defines Maximum - that is the FIELD gauge. If they define NOGO, I have not found where they do that. So far as I know, there is no SAAMI "head space gauge" for cartridges - they either fit to the chamber (within tolerances) or they do not. I suspect some modern marketer has invented a "headspace gauge" for a cartridge case, although I think that is misleading nomenclature.
