My younger daughter does this and has since she was two. It wasn't until we learned she has trouble pronouncing several sounds that we realized why; she simply chooses the word that is easiest for her to say even if it isn't the best. They still fit, they just seem odd coming from the mouth of six-year old with a speech impediment.
Interesting. I just assumed DS9's vocab was different simply because he reads so much. He's always had a unique way of saying things. But now that you mention it, perhaps he said he had to "adjust his eating habits" when he lost his tooth versus "change the way I chew my food" because the "ch" sound was one of the sounds he didn't have until after speech therapy.
