Google has an engineering office in Austin. Could be someone who took their Bay Area salary with them. Same with Pittsburg. Uber has a presence there and SFBA transplants take their SFBA salaries with them when they move.
It's usually not, but there's a certain logic to it: they valued your work high enough to pay you in SV, why would the same work be less valuable just because you moved offices?
Austin is not SV, but rents there have jumped in the last decade. Try pricing out a 2br apt, near one of the major tech offices. First place I clicked on was $2,300 a month! Average is closer to $1,500. This is unusual for Texas =)
300k is definitely highly unusual for Austin. With that salary you can literally live like a King. Buy a house in the city AND one on Lake Travis for the weekends.
levels.fyi shows people making $300k in Austin, TX (which as far as I know, is not a high cost of living area)
What are some areas that are comparable to South Florida that are listed on levels.fyi?