I've known someone else a bit like this, where he seemed "stupid" - found it very difficult to learn traditional academic subjects - but he was top notch at manipulation, and had an uncanny knack for guessing what people wanted in any given scenario.
You can't be truly dumb and successfully manipulate the majority of people you meet. Put another way, people who are as limited as he seems - find it very difficult to understand people and get what they want/need from them.