Interesting question. I've just scanned over the first few paragraphs of each story, as I have the whole volume of all 60 stories.
you're right, Watson does narrate both stories similarily, I never noticed that!! But watson chose different words in each story, to demonstrate the same thing, which is Holmes amazing talents. Plus, at the time of writing these stories, Watson was a little bit wary in what he wrote, since Holmes was very critical of his writings.
good observation!! this is the best way I know, to answer your question.