Marketing is what marketing is.
There were several styles of rifles (and shotguns) produced by the Hawken Brothers, Gemmer and the other owners of the Hawken Shop. The architecture of the rifles throughout the production of all the owners varied from the classic plains rifle / mountain rifle to a scaled back "California Rifle" and a scaled back squirrel rifle with brass hardware and a single tenon that looked very much like the T/C rifle. Local rifle makers including such as Dimmick, Creamer, and several others also made rifles to fill the needs of those using rifles in the west, locally for hunting and target shooting.
I would speculate that Investarms wanted to still be able to sell the rifle that they had the manufacturing process and needed a name that would not infringe on Lyman's rights to the Plains Rifle trademark with a historic name that would generate more interest and relate better to the architecture than a catalog number such as Model 160 and Model 170.
Muzzle Loading Rifles Investarm