It's not just in the math sense. Everyone I know who's met him (including some of the very smartest people I know) have been very impressed by his ability to quickly grasp the fundamentals and even the minor consequences of a particular idea or action.

Actually I wasn't :-) What I thought funny were (i) the concept of futuristic fax machines (do people realize that most of these still have internal modems at 9600 baud?) (ii) the naiveté of the term "screen phone", it's so much more charming than "smart phone", and (iii) the description of it as a "phone with a minicomputer". Minicomputer indeed, today's 1GHz phone processors are more powerful (and probably have more memory) than the Compaq laptop Gates was using then.