The California ballot had two Democrats and no Republican for US Senator.
That alone can fully account for the difference.
Yes, I know only too well. Lefter and Lefterer. But this article references only the House races. In a lot of California districts, the incumbent Demorat faced token on no opposition from any party. I also noticed while looking it up that, in Massachusetts, 4 of the 9 Congressional Districts had the incumbent 'rat running unopposed.
Says a lot for the future of our representative republic, doesn't it? :(