I think one of the mistakes people make is assuming the central/northern divide is made horizontally.
Here's an example as you see the california government's designation of northern central california. You'll notice sacramento is the southern point right at the border with central california and the map for northern california is a separate one.
Here's the map of the bay area which is generally lumped in with northern california due simply to the convenience. It's technically not norcal either but it's regarded as such. A lot of it has to do with the fact that as a counterpoint to southern california, the bay area is a economically and socially very active area, so it's countered with southern california. Despite sacto being the state capital central cali just doesn't have a lot of relevance in other ways. So in the past when people talked about either north or south cali they talked about things like the black panthers and just said they were from northern california even though "the bay area" is a more accurate, specific description. But in reality that's not the only reason.
I know this stuff is probably boring (I did a lot of political work so we had to learn the counties and boundaries etc. so i'm probably the only who finds this interesting) but Sacramento is part of the Central Valley and central valley is a term that's usually associated with central california.. As wikipedia says
Because of the ways people tend to speak of california in sections sacto is viewed as part of the central valley and that ties in the areas you're talking about like Fresno. The reason this is notable is that even though sacramento might be norther of the bay area in terms of a straight horizontal line on the map, that technically is not how the districts are regarded in large part because of the shape of the valley. The central valley sort of runs up the middle (as the name suggests) and the bay area which sits to the left of it is then grouped often with northern california because neither of these is part of the central valley. AS a result northern california is actually a kind of horse shoe in the way people generally refer to it.
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I figure the government is the best source for explaining it. For those not from there it's explained here on the department of transportation's site. I know it's confusing.
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