It isn't that your understanding is simplistic. Rather that the interactions of the plates are rather complex, and have evolved through time. "Triple Junctions", which is what we are talking about here, are sometimes tough to visualize in a simple diagram, let alone describe in a short text message.

I knew I'd seen a nice animation somewhere, and after a bit of googling, I found it. Check out "An Animated Tectonic History of Western North America and Southern California" by Tanya Atwater. It shows how over time the situation changed from one long subduction zone, into a subduction zone linked to a major fault (the San Andreas). The Mendicino Fault is not labeled in the animation, but it is the prominent East West trending line that eventually colides with N America. There is a bit of text below the video panel that helps explain it.

(Back when people were just beginning to wrestle with the implications of Plate Tectonics, Tanya Atwater was one of those who had a "Eureka Moment" about what it meant for California.)