How can I not get a joke in my dream? The Dream

Today I had a dream. I was in class where I and my classmate were chosen to be examined. Once seated in front of the teacher we got our questions. She accidentally saw mine and made a funny remark about it. I did not get it at first and so I just smiled in a general way. Since in real life she is used to me getting her jokes she immediately explained it to me. Only then did I understand it and had a good laugh.

The Question

How can I not get a joke made de facto by myself (even though on behalf of someone else) in a dream?
It maybe the joke was the dream-equivalent of a MacGuffin, a fuzzy, unimportant placeholder that wasn’t materialized into actual language, and that the point of the scene was to experience awkwardness, embarrassment, inferiority or exclusion that others had the upper-hand with in-group knowledge unavailable to the subject?

It would fit that what seems real in a dream isn’t necessarily real... that the idea of certainty came from the dream while waking up, but the actual content of that certainty likely never existed.