Indeed, glassdoor is bad (estimates low), at least at Microsoft, Google, Apple, and Facebook. I saw it on two fronts; as a manager, the salary ranges are fixed for each of the title/pay grads.
As a hiring manager, human resources also paid for Serious Surveys (i.e., not just relying on the people who enter their info into a random website) of salaries for comparable jobs at other places. Of course, that was probably superfluous, as the HR and recruiting people swap between the big tech firms pretty frequently (seemingly moreso than engineers), so for the cost of a trip to the cafeteria starbucks, you could usually get info on comparable salaries from a recent transfer.
As a hiring manager, human resources also paid for Serious Surveys (i.e., not just relying on the people who enter their info into a random website) of salaries for comparable jobs at other places. Of course, that was probably superfluous, as the HR and recruiting people swap between the big tech firms pretty frequently (seemingly moreso than engineers), so for the cost of a trip to the cafeteria starbucks, you could usually get info on comparable salaries from a recent transfer.