Most gas standards in the gas industry are hugely inaccurate because the stability of the cylinders is so hard to get right at scale. Problems at primary manufactures of specialty gas cause problems all the way up the chain to NIST. For years there was no SRM for ppm H2S mixes.
Yeah it's decently crazy. They set the accuracy for oil and gas, water treatment, etc safety monitors and alarms. It's a constant war to maintain quality and accuracy