I can't understand the sub-genre "Bay Area"

So far I only know that it's a subgenre of west coast rap and that the major exponents have been: Too $hort, E-40, Tech N9ne, Dru Down etc. But why is it diversified from west coast rap? What substantial differences does it have with it? Listening to Tech N9ne's album (Calm Before The Storm), for example, I don't notice any difference with the west coast genre in terms of sound.

Help me to understand the story of this genre