Cannot vs. Can not

Have you ever thought about the difference between the two? No? Well, that just confirms the fact that I'm strange, so nothing new. But anyway, this is one of those things that makes me think the English language is whacked out. How do people learn English? Back to the point: you see, one means 'incapable of doing' and the other means 'has the ability not to'. It's such a tiny difference, it doesn't really matter but still. I wonder how many words there would be if we broke down all those tiny inconsistencies into new words. Far too many is the answer you are searching for.