2 and 3 saying they don't know eliminates some uncertainties 1 had about their own number (any combination where the other two would see numbers that could tell them their own). After those possibilities were eliminated, the 1st person has narrowed it down enough to actually know based on the numbers shown above the other 2. The puzzle could instead have been done in order 2, 3, 1 and 1 would not have needed to go twice.

I guess really the only missing information is that they have the exact same information you do, plus the numbers above their friends heads.