jstoez
One thing you are completely missing is that around here people are far more likely to age in place than other parts of the country. No need to retire to a warm climate, they are already here. No need to move because property taxes went up, Prop 13 takes care of that. House too big for you, just keep it because housing is expensive here, tehre is a steady infulx of new residents and you can easily rent it out for a nice amount.
In many nice ZIPS in SD county you have a lot of homes owned by low income retirees who want to and can afford to stay here. That keeps supply low and helps skew income lower tahn it would be if you looked at who was moving in rather than everyone who already lives there. When you get down to it, the median income for a ZIP is pretty meaningless in real estate. What matters is the incomes of the folks who want to, can afford to and actually buy a home there.