Not necessarily lie. People, that earn less, are likely to be ashamed (even anonymously) of their salary, while big earners are prone to boast on every occasion.
How do you know that the data on Glassdoor is too low? What is your source of truth? Anecdote? Glassdoor currently reports the average base salary for the job title "Senior Software Engineer" in SF Bay Area to be ~$130K which sounds about right to me, again, with nothing but anecdotal truth reference.
If you go by what people say on HN, then of course everyone in software makes $300K a year and has millions in stock options (and gets 5 job offers in their inbox a week).
I know how much I am paid, how much some of my coworkers are paid, how much my company pays new grads, and what the average salary on Glassdoor is for my company. The average is lower than what new grads got in 2016.
On top of the incorrect base salary, the information on bonuses/equity is extremely limited. The average that is listed is also incorrect.