Judging by the footnote, it's YouGov which isn't a very good poll typically. That said, it's likely got enough people across ages to standardize it properly. They probably do have a larger amount in one demo vs another, but you can simply weigh them differently to balance it out.
There's probably plenty wrong with their methodology if we dig deeper though; these polls aren't very scientific typically. With political division, it could be how they were asked, for instance.
Man we have way too many people on the ballot at it is, all you need is $3k. I mean, literally anyone runs, I was reading the booklet and there's even a guy named LivingForGod AndCountry, a Barack Obama, and some very deranged conspiracy theory guy who earned a little asterisk saying his views don't represent the office.
The problem is, without big money, nobody stands out. You can't just rally for some rando-- they're already trying that, and it doesn't work. CA Governor Primary is one of those situations where it was well intended but in execution it falls apart, especially after Citizens United.