As far as I am aware, the data powering these sites (and HN's comments) are voluntary and self-reported, and there is no correction for this self-selection bias. Few people making average or below average compensation really want to admit it or post about it in public, so you're only going to see evidence of above-average salaries, biasing the resulting average figures.

Senior devs don't get those numbers on average in the bay either. Facebook/Google are the top (NFLX too but they only hire senior engineers) and few other companies close behind like amazon, apple and some fintech. I'm saying you can get those numbers because I myself am making 5% below those numbers (245k for mid level) in Chicago for what levels.fyi is showing what FB is paying for their engineers.