I do the same thing when typing, but not when writing by hand- I have always wondered why.
My theory is that typing is simple enough, that it becomes pure trained reflex. That is, it gets done without conscious thought, just like talking, or walking. You don't have to think Writing is harder, and slower, allowing your conscious brain a chance to "look" at what you are composing. This is especially true if you want someone else to be able to read it. :)