Isn't this the very reason that homework assignments exist? When I was in college, it took me two years to realize that I could have easily gotten As and Bs (instead of Bs and Cs) in my various math classes, physics, chemistry, etc. had I simply bothered to do my homework properly--or, to put it differently, had I only properly applied the knowledge which I passively acquired by reading the associated textbook sections. Notably, I was always convinced that I "understood" everything I had read, only to find out otherwise when I tried to solve any of the problems. It was only after I actively applied the knowledge I had picked up, that my understanding transitioned from superficial (as in, understanding the underlying logic itself) to concrete (being able to apply the logic toward other problems).
If you apply this to abstract and esoteric technical concepts, it is easy to see why someone might say they understand--and even believe that they do--while in reality only having a superficial understanding at best. The problem then becomes getting the other person to spend the effort to properly internalize the concepts being conveyed, before they have built up the requisite interest in the idea to be sufficiently motivated to carry out said effort.
It's probably also true, however, that this may only apply to sufficiently esoteric and complex ideas in the first place.