Everybody gets taught phonics. That means they attempt to spell words they have only heard the way that they sound (which also depends upon the person saying them not having an accent/knowing how to say them properly themselves). Sometimes they'll get it right, sometimes they'll get a word that could feasibly be pronounced in the same way due to the flexibility of English, sometimes you'll get something where, if you switch off your WHAT THE DEVIL IS THAT MEANT TO BE? response, you'll realise that they've spelled the word absolutely correctly if you're using phonics.
Hence 'hun', 'vowels' instead of 'vows', 'spade' instead of 'spay' - and then there's Autocorrect on top. If you start the word off incorrectly because you've never seen it written, it's not guaranteed that it can pick up that you actually mean something starting with completely different letters.