Because, unless he's saying them with other Catholics, when he says the 'common prayers', he uses he forms he first learned, and so the words are sometimes different. Hence, "Glory be to the Father, and to the Son, and to the Holy Ghost", and "Hail holy queen, mother of mercy, our life, our sweetness and our hope, all hail!". And that's without getting into the creed. Things like "Was crucified, dead, and buried" or like things.Minor differences, yes, but I got a talking-to about how we're Modern Catholics and we don't use that name for the third person of the Blessed Trinity when I, without paying attention, didn't use the modern wording once.
But you know, you get into patterns, and almost nothing can change them. But if you think that's bad, you should see the egregious abuses that I commit during mass.