In my view it is not difficult at all.
German military called this caliber 7.9 mm. Other nations who bought armament from Germany (like China, for example) knew this caliber as 7.9 mm.
German civilian designation was, I simplify somewhat, 8 mm. So commercialy it was known as 8 mm. SAAMI even standardized it as 8 mm Mauser, although Mauser had no role whatsoever in the creation of this caliber.
After the end of WW1, Czechoslovakia obtained a lot of machinery from Germany and started production of German design rifles and ammunition. For some reason, they decided to call the identical caliber 7.92 mm. Because the Versailles treaty had removed German industry as competitor and the quality of the Czechoslovak products, the 7.92 mm figure for the German 7.9 mm caliber found widespread use.