It's only Saturday morning! How do the even know yet?
They won't know 100% until they do the weekend totals obviously but they have years and years of cinema data to work from so they extrapolate based one past movies based on what was earned on Friday/typical drops against the Friday drop/typical weekend attendance differences things like that. Unless this movie has a huge Saturday upswing (which I believe happened with Force Awakens it got a pretty big weekend bump after the estimates) or something, the estimate is probably pretty close.