I have a theory that Glassdoor, Indeed, etc., skew low on purpose. They want to retain business partners and businesses, more often than not, want to pay the lowest amount possible for labor.

There's also the growth factor consideration that probably isn't implemented by those websites. As in, salary data from several years ago might be not be adjusted for inflation and have the same weighting as recent salary data.

Half the time, the websites can't even gather salary info from job posts and rely on estimates despite having hard data.

What this can do is that less savvy users might not notice the estimated data tag and use the lower range in negotiations, which could ultimately lower starting salaries for workers moving forward.

I've written job boards before about this, but of course they aren't going to change anything as it isn't in the their best interests.