> Probably because "If you can't explain it simply, you don't understand it well enough."

People consider different things as simple. For example I often consider very abstract mathematics as very easy to understand (since I always thought in a rather abstract way - thus this kind of explanation is very natural for me), while most people consider my kind of explaining things as "unnecessary complicated".

Don't tell me that I haven't understood the topic. When talking about topics that I understood worse, I often tend to give much simpler (and better understandable for "ordinary people") explanations, since I haven't understood the complex connections and thus I'm unable to give a much "simpler" (in my terms; "more complicated" in terms of most people) explanation.