Because Fastback and Roadster timelines are not the same at the beginning so the mark numbers don't align. The Roadster wasn't introduced until approximately 3/70 lasting to approximately 12/70 after which it became Roadster II from 1/71 to 12/71 when it jumped to Roadster IV in line with the Fastback IV and new Interstate. So technically the MkIV Roadster should have been MkIII.