@OP: That's exactly what I was thinking, too. People often don't even seem to be aware that they have been told something that they are expected to understand. I often wonder how much of what I say even register in their head. They don't seem to worry whether they have understood something or not because they aren't even aware of it.
So many people are overconfident and they don't even realize they don't understand something. It's in a little different context. When I said to some other students that a certain subject (at uni etc) was very difficult to understand, they all said that that subject was very easy. However, the exam results proved that they hadn't actually understand it that well.
I tend to worry a lot about whether I have understood something or not. I always feel I don't understand many things and because of that I can't feel confident in many situations.