I get a similar thing quite often (FWIW, I'm not dyslexic). I think that part of it is that when we're typing, our fingers learn a "muscle memory" of certain key-presses, so that even if we're thinking of the right word and know how to spell it, our fingers just get stuck typing the same pattern each time we try to type a certain sequence of letters. It happens more when I'm tired/stressed or when the thoughts are coming faster than I can type them, so that I'm relying more on the muscle memory "autopilot" instead of concentrating on the letters on the screen. My most common one is typing extra "i"s in certain words which have consonants alternating with "i"; for example, "existing" often comes out as "exisiting". It's very annoying when you're a coder and have to get all of your variable and function names letter perfect!
_________________
When you are fighting an invisible monster, first throw a bucket of paint over it.