One has to remember that all SAAMI specifications are 100% voluntary, meaning they aren't carved in stone and there's no penalty if something doesn't meet their established specs. As has been discussed here ad nauseum in the past, you won't find a factory barrel that has a chamber that's to SAAMI spec as every manufacturer has their reamers made to their desired specs (usually with longer throats/freebore). Headspace gauges often differ in length from manufacturer to manufacturer based on how they decide to spec them within the SAAMI specs.

For instance, Fred headspaced the .284 Win barrel on my Axis with his Go gauge after T&T'ing the action (not sure of the gauge brand - but I'd guess PT&G or Dave Manson). I happen to have a set of JGS .284 Win gauges from a past project series we did here on the site, and when I try to close the bolt on my JGS go gauge it won't close indicating the headspace is too tight. Like I said before....tolerances. The gun shoots fine and my FL sized brass chambers fine so I see no need to mess with it.

The below is an excerpt from a document put out by Dave Manson Precision Reamers (link):

Confusion often arises when comparing headspace readings generated by gauges from different manufacturers--a rifle may accept both “GO” and “NO GO” gauges from one maker but only the “GO” from another. This would lead a reasonable person to ask, “why?” The answer lies in the dimensions and tolerances to which the gauges are made.

It’s important to understand that the various makers of headspace gauges do not use the same dimensions for what is termed “GO”, “NO GO” and “Field”. We know the specifications used in the manufacture of our gauges, but cannot speak to those used by other makers--except in very general terms—so the information given here applies only to Manson-manufactured head space gauges.

We use both a two-gauge and three-gauge system for checking headspace. Three gauges (“GO”, “NO GO”, and “Field”) are used with rimless, shouldered rifle calibers, eg. 30-06. This is done because SAAMI allows .010” between minimum and maximum headspace for these chamberings. Many feel (ourselves included) that only knowing headspace within .010” is imprecise and that an intermediate gauge is called for.

In rimless, shouldered calibers, our “GO” gauge corresponds to the SAAMI (or industry) minimum; “NO GO” is ground to minimum plus .006”; “Field” corresponds to SAAMI maximum (min. plus .010”). Our manufacturing tolerances are such that actual gauge measurement will always fall within SAAMI limits.

We offer two gauges as standard on all other calibers—shotguns, rimfire, rimmed, belted or rimless pistol(45 ACP). In this system, “GO” corresponds to the SAAMI (or industry) minimum and “NO GO” to SAAM I maximum. Again, tolerances are such that actual gauge measurement will fall within SAAMI limits.

We do make headspace gauges on special order to specifications other than those outlined above. All such gauges are clearly marked and are easily differentiated from our standard product.