However, during the early days of the European explorers, nobody bothered to do any of those things, because there were native peoples to rob and murder, and proper cartography simply would've taken too much time. After all, priceless religious artifacts don't melt themselves down into Spanish doubloons. Consequently, California was incorrectly drawn as an independent landmass for over 200 years.
You see, California was first charted in 1533 by Fortun Ximenez, a mutineer who broke off from Hernan Cortes' original Aztec-busting fleet. Ximenez took his stolen ship north along the Pacific coast of Mexico, and wound up landing in Baja. He decided that he and his rebellious shipmates had just discovered the Island of California, despite the fact that there was absolutely no evidence to suggest that the land he'd just stumbled upon was actually an island. Also, the Island of California was a fictional place from a famous Spanish novel, which by definition would make it difficult to locate in a hijacked galleon. At any rate, Ximenez had no time to retract or amend any part of his declaration, because he was promptly killed by natives.
After receiving word from the survivors of Ximenez's crew, Cortes took some ships up to Baja himself and backed up the mutinous lunatic's claim that California was indeed an island separate from the continent they had just beaten the ancient shit out of, most likely because Cortes wanted to establish a new colony he could call dibs on governing.
The Spanish government supported Cortes (as it usually did) and had a ton of maps drawn up with this hilariously flagrant error. As a result, maps all over Europe showed California as an island, including those four random and completely nonexistent lesser islands in between California and the mainland that somebody threw in there for no conceivable reason. Other explorers, such as Juan de Fuca, continued visiting California, but they kept finding inlets they didn't want to travel down. Rather than waste their time doing any actual exploring, they would all simply announce "Yep, it's totally still an island" and go on their merry way.