Authors are such lucid and beautiful writers when they don't realize they are writing.
One of the great joys of editing work in fields I know nothing about is asking questions of my clients. The answers, over email or over Skype, are unfailingly clear, logical, and fascinating.
There is something about explaining deeply understood concepts to someone smart but without expertise that brings out the best in writers. Their prose becomes so clear and so verb driven (see Lanham's Revising Prose for more on verb-driven prose). I think this is probably what lies behind the academic tradition of having researchers teach, in fact--explaining complex concepts to neophytes is a fantastic way to distill your own understanding down to perfect clarity.