A similar problem happens on Glassdoor. If you look at the salaries for pretty much any position at a big legacy company the bulk of the data comes from reviews submitted up to a decade ago...during the depths of the financial crisis.

At many smaller companies with only a few submitted salaries, the problem is much worse. I've received offers for 2.5X the quoted salary for the same position glassdoor supposedly had 10 submitted salaries for (bay area).

None of these review aggregators are incentivized to dump old reviews and make it look like they have less data than their competitors. Inevitably, consumers will start trusting reviews less and less...similar to what has happened on Amazon.