Sometimes when I am typing my hands will type something different to the intended word. Why?

It's just muscle memory. Your hands are used to performing one combination for the more common word, and when you think of typing a word close to that one that you haven't had the chance to type as often, your hands resort to what they remember (figuratively of course). If you start doing this too often I suggest you just slow down and concentrate on the less common word you're trying to type. It makes it easier to use it again later. Soon enough, you won't need to slow down or even think about what your hands are doing as you type it.

i do this sometimes. I should think its pretty normal.

Unless it happens really often then it could be a problem and you should consider talking to someone about it.