Please explain the email scraping from twitter

I've been reading about allot of email scraping tools, where some of them scrapes emails from twitter. What I don't understand if it scrape emails that the twitter user have displayed somewhere? Or does it scrape the email the user have choose to signup with?

I think it's the emails they posted in a tweet.
I could be wrong though, but it would be a big security flaw if it was the email they signed up with.

Emails posted in tweets or in bios, am looking for a script myself