I found no logical explaination but it appears both are acceptable: https://www.miniatur-wunderland.com/discover-wunderland/technologies/others/hints-tricks/model-trains/gauge-h0/.
According to Wikipedia: "The name for O gauge and O scale is derived from "0 [zero] gauge" or "Gauge 0" being smaller than Gauge 1 and the other then-existing standards. It was created in part because manufacturers realized their best selling trains were those built in the smaller scales."
Simon