I'm an excellent speller and a terrible typist. Ask me how to spell any word and I can probably spout it off. But set me in front of a computer and I make a mistake on every document I write. I wish I could blame it on my keyboard, but I regularly use three different computers and I make typographical errors on all of them.
One interesting note is that I seem to make different errors on different keyboards. On my laptop, which I do the bulk of my writing on, I have a tendency to miss keystrokes. That's why "that" becomes "hat" and "one" becomes "on". Typos like that you are far more likely to see because they happen to be real words, so my spell check doesn't flare up at it and I don't always catch it.
The other computers are desktops, and on those keyboards, I am far more likely to hit the space between two keys and push them both down. I always know when I do it, but it is annoying and I have to take the time to go and fix them.
Regardless of how they happen and what kind they are, typographical errors tend to occur for one reason: rushing. They always happen to me when I am trying to write faster than I can. Sometimes that is unavoidable; your mind works faster than your hands, so sometimes they have to catch up, especially when you're on a roll. But if nothing else, know when you are most likely to be making them.
Typos are still inexcusable in any important document or communication. They reflect poorly on you and make you look dumber than you are. The more that make it through, the worse you look. Take the time to catch as many as you can. I can't decide which is worse: typos that I see in published materials like books (which are few and far between), or the dozens of typos I see in emails on a daily basis. In either case, they're both awful and you should strive to make as few of those mistakes as possible your own.