This is because a high percentage of California voters are vote by mail. When a VBM ballot arrives on or after election day (it can arrive 3 days after if it's post-marked by election day) the ballot must still be verified as being from an eligible voter. This can be time consuming because people who vote by mail can show up at the precinct on election day and vote provisionally. Thus, the VBM ballot has to be checked to see if the person voted provisionally. Likewise a provisional ballot has to be checked to see if the person did vote by mail.
California had 8.9 million ballots counted on election night and now has over 14 million. In the past, ballots counted after election day skewed heavily Democratic. I talked about it in 2014. That year Democrats did better in 30 of the 31 races I tracked. This year I tracked 55 races and Democrats did better in 42. So Democrats doing better after election night still holds in California. I've said in the past that a Republican needs at least 52% of the vote on election night to ensure a win. There were only five cases of Republicans losing more than that. The biggest change was in AD-36. Tom Lackey led by 12 points at the end of election night and only won by 6.