Exit polls that came out before the results came in showed Hillary winning California by close to double digits. And that is ultimately by how much she won California.
Obama tends to overperform in exit polls because older voters are less likely to respond to them, because they're now generally conducted by college students.
The early California exits had Clinton up about 12, IIRC, which is notably more than the 8 she got.
And again, please show me a source that substantiates your claim that early voters are measured in exit polls (how? By mashing phone interviews into exit poll interviews?).
But that would mean that Clinton, in fact, was overstated in the exit