This is a question concerning technical part of a project. Sorry for your waiting, we had to contact the dev team.
In the most cases, we guess emails. We have a big database so when we know the pattern of a certain domain we know exactly what email would the needed person have. After we guess, we verify the email and then give it to the user. Sometimes our guesses are wrong, but we still show that result.
So we need contributors so we less guess and know for sure. That's where we are heading to with our token sale.
Good to know this! Actually, I think those are oly minor mistakes. The extension is really good at finding leads for websites where even Hunter fails. I personally am quite content. Also, if the extension happens to find emails that are no longer valid and functioning, it highlights them as invalid by marking the email with a red circle instead of a green one.