No need to apologise: I just read that with a weary sigh because the only place I make spelling mistakes is when on the internet. It's always homophonic pairs e.g. hear/here, to/two, and even through/threw.
I've found the explanation for this in the area of psycholinguistics (I'm working on a thesis at the moment) and it seems to be do with the processing mechanism for speech generation (although in this case the key difference is that one is generating typed words). What seems to happen is that you trigger the search for a word but the wrong homophone has a chance of winning out and it is usually the commoner one of the two as far as I can see i.e. of the two words, the one which occurs more often in everyday use. The really interesting thing is that I never make such mistakes when hand writing, so I think it is down to the mind having to pay attention to typing (however subconsciously: I'm a ten finger typist and don't have to think while doing it) while simultaneously seeking out lemmas (the posh term for pre-words as stored in the brain). What I suspect happens is that the mind, being distracted for milliseconds at a time, sometimes fails to filter the wrong homophone out.
I hope that makes sense. And if any of you have kids at school who need an explanation for failing spelling tests, feel free to print this out so that they can learn it and spout it to the teacher.