It wouldn’t be surprising to learn that Bush is carefully trained to say “nucular” and “misunderestimate” and the other silliness that intellectuals like to ridicule. That’s probably about as real as the ranch constructed for him, and the rest of the folksy manner. After all, it wouldn’t do to present him as a spoiled frat boy from Yale who became rich and powerful thanks to his rich and powerful connections.
So Bush is created to be an ordinary guy with his sleeves rolled up and you could have a drink with him in a bar. My guess is he's taught to make those mispronunciations and grammatical errors; I doubt that he talked like that at Yale. He's probably taught that way so that 'them liberal intellectuals' would make fun of him and then they can say, oh yeah, he's an ordinary guy just like you, going off to his ranch.