If, on Mastodon, you make a DM between you and someone else, then you @ a third user in that DM, then the third user becomes able to see that DM. At least that’s what I heard a few weeks ago.
This is because DMs are, regrettably, just normal posts with default visibility of two people. If you @ more people, then they gain visibility into the DM.
That 80% is important. We need non-techies, because they remind us that there’s more to life than just computers.