I understood the monad concept for a few months in university. After the exam was over, I soon stopped understanding it. The same thing happened with the concept of VC dimension. It's kind of interesting, because we usually don't think of "understanding" as something that comes with a time limit.
It happens all the time. For a brief period I understood musical notation and rhythm and then it was gone. Similarly I had a time in my life where I knew by feeling whether a French noun was le or la.
The sneaky thing here is that understanding, and knowledge in general, disappear silently. So you don't notice it when you unlearn and forget something. Only if a situation comes up where you need that understanding again do you notice that it is gone. Coming to understand something is conscious, losing that understanding is unconscious.