I know, it's kind of an inside joke to some long time San Diegans, like my aunt, who taught it to me. 1 and US 101 are both referred to as PCH to old Californians because in SD they were. But in San Diego they only have the U.S. 1 now so there's no confusion. The 101 was decommissioned in SD county and the OC, but for some stupid reason people in SD county never really caught on that the OC didn't have the 101 either. Hence, the 101 starts in the OC. Adding to the confusion, some towns like Del Mar and Solana Beach, where my aunt and parents live, they still have historic highway 101 plates (brown and white) hanging around on random sign poles.
But like gm 2 was saying, San Diegans can get away with that b/c they don't live in LA where the 1 and the 101 are about 10 miles apart,
If you google US Highway 101 and click a variety of links, you will be throughly confused. The first two links from "Guide to the Pacific Coast Highway", and "Wikipedia" offer conflicting info right off the bat. [laugh]