I don't think they just pull 125k out of the air. I think that at 90k you are not going to get many great candidates. And at 250k you risk spending a LOT more money and still not being able to successfully recruit people that are much better than the 125k people. It is not like all your 125k people are going to quit when you double salaries to make way for all the FAANG people who are looking to switch jobs for the same salary.
I think that hints to how these pay bands come to be within a company though, it's not a transparent and liquid market with a bid/ask spread and a clearing price. In practice the company decides a band that they think gets people in the door. HR gets involved, and management settles for a 0-2% inflationary increase YoY rather than the marginal rate.
If you combine this with one employer bordering on a monopsony for buying a particular category of labor - then pay won't move in proportion to productivity.
