The difficulty with that is that trying to define "true understanding" crosses deeply into the philosophical realm. Moreover, a lack of ability to define a term does not invalidate my position. That being said, I'll instead present you with a question intended to display what I believe is the difference between the two. If I write an example of an integral, it clearly shows that I know what an integral is, but does that mean that I understand integrals? Even if I could concisely answer any question you asked about integrals (i.e. an all-knowing AI), that is not the same as my definition of "truly understanding" integrals. To pose another philosophical question which highlights what I believe it is to "truly understand" something: Does the library of babel understand? I would say that despite it having the answer to any question you ask, it does not "truly understand" anything. I wish that I could put the difference between these two into concise wording, but alas I've been unable to quite put my finger on it despite having spent some good part of the day giving thought to it.

Some of these concepts are explained/explored much better than I could do on the wikipedia[0] page for understanding, particularly the "Assessment" section.

[0]https://en.wikipedia.org/wiki/Understanding