I would guess that the difference native/foreign is simply due to the way language is learned: for native speakers, it's first and mostly orally. This doesn't explain a later appearance of mistakes though…

As a nonnative English speaker (actually mostly reader/writer/listener), I started doing that at some point (many years after English proficiency), to my own dismay.

I'm not native and I absolutely do this. 500 grams of flower...

> I don't remember ever making this mistake (but do remember plenty others to make up for it), and can't imagine myself doing it. Yet it happens to native speakers all the time.

I used to think that, too. But now my fingers just type the words as I hear them spoken in my mind, and that seems to occasionally produce homophones.

Kinda fascinating what this says about our language processing, to be honest!

I was most disconcerted to find myself doing the same thing of late. It is very curious; like my brain internally just couldn't be bothered anymore to expend the energy to delineate their and there until I'm in the process of actually typing. But that means the signalling fires a tad late, so I'm going back and fixing stuff.