There are some variables which might affect how and why it happens. For me, there is a big difference between tapping a message, letter by letter, on the screen of my iPhone - which is a slower and more deliberative process for me (hence less 'mistakes') - or writing it on the excellent keyboard that governs my Mac computer. Incidentally, I am fairly fast a 'touch typist' - and also a writer by trade --- and I vastly prefer typing on keyboards to writing by hand... because it's faster. My brain works at a certain speed and when I'm obliged to print out or write things out 'cursively', it's always felt frustratingly s-l-o-w. But typing - and typing quickly - feels closer to approximating the way I think.
But then, rereading - I see mistakes, missing words, misspelled words, incorrect words - a whole gamut of stuff which I doesn't seem possible.
My overall theory - whether texting or writing by hand or even typing - is that a) our brains function at different speeds than our hands do, and b) though it 'sounds right' in our minds, stuff gets 'lost in translation' ('translation = the art of putting thoughts or ideas down in words).
I think it was John Updike who said, "writing is rewriting" - although doubtless many other women and men have expressed similar sentiments - and those of us who use words (a group which includes much of the human race) all have to do it (rewriting) on a regular basis. And not just to improve it or to inch closer to some ideal of how we want the words and phrases to sound - but often to rectify and deal with exactly the kind of stuff you are describing, Andrew.
It's a big club and I think I belong to it, too.
Or, as Birar siad soo elbowqwintly, as lnog as one deonst mcuk up the fsirt and lsat lteerts, ploepe wlil unstanerd.