Same thing with the coax we use. RG comes from an old mil-spec (MIL-C-17). Of course, in order for a cable to be properly called RG cable, it must meet the MIL-C-17 specifications -- no more, no less. While the electrical specifications and most of the physical characteristics of the cable we use meet those of RG 6 (M17/2), the shielding is different than what the specification calls for. Different manufacturers can use different components for the jacket and/or dielectric, and/or make the jacket different thicknesses. All of those are differences from the spec and technically cannot be called RG cable.
All of the major coaxial cable manufacturers correctly call their cables as "RG "X" type" or "Series "X" cable". Same with the major connector manufacturers. They call their connectors "Series X" connectors.
But, we're stuck with everybody saying RG "X" cable and RG "X" connectors.