To put it simply, the entire state of California (like with Washington) has a minimum sales tax. That number gets bigger in different areas because they'll have different and/or more needs than other places. Counties can add to the taxes, as well as cities.
i.e. In Vancouver, the sales tax is 8.4%, whereas in Seattle it's 9.5%. Seattle has more needs to be met than Vancouver, much of which includes road work, police, firemen, city councils, etc.
Get it now?