I’m not sure.
I’ve been writing creatively, as well as through very extensive personal correspondence, all of my life; and I found that upon the transition from writing on paper - or even on an old-fashioned typewriter - to writing on the Internet, the tendency to confuse homophones greatly increased in Internet writings. I first noticed this on Usenet more than 20 years ago; people whose educations and erudition were apparent would frequently make those same mistakes.
I think some ‘brain-thing’ happens, or is different, when writing to the internet/on a computer, even with people who, for instance, clearly know the difference between ‘who’s’ and ‘whose’.
I’m not sure why it’s true; but my ‘working hypothesis’ is that “talking” to one another, in the way that we are doing now, is more like ‘speech’ than it is like ‘writing’. Our brains are getting confused by the conflation of two different means of communication.
-JT
Ones that drive me crazy are using ‘then’ instead of ‘than’ and ‘their’ for ‘they’re.’ Or vice versa.
I think that you are right about your theory that it is easier to do when you use the computer and are posting to others than it is when you are just writing for some reason. Whatever the reason it might make an interesting study about the brain and communication.