Yes - I frequently make the their/there and no/know substitutions when typing and theirs simply know excuse for that. Luckily I usually spot such errors before sending/saving the text, but I have been known to edit a forum post to belatedly put them right. Sometimes, of course the error is spotted too late - an e-mail that's already gone off or, much worse, when it's in a printed publication. I make word-ending errors too, but in my case it's usually omission, as in "fluff" instead of "fluffed" or "fluffing", rather than omission.
As with you, these are not the sort of errors which I would ever make when writing, so I too wonder why it should happen on a keyboard. I suppose it could be speed - do we type faster than we write, so fast that we're forced to think of what's coming next rather than paying attention to the current word?