In may places this has been archived as a blunder on Doyle's part.
I've also come across some interesting idea regarding this...I'm
quoting this here:

"In some of the stories about Sherlock Holmes, Dr John H. Watson is
referred to as James Watson. The reason for this is unknown, but one
theory is that the good doctor used the name James when enrolled in
the military.
This is not the only discrepancy however; it seems the wound that
ended Watson's military career has a tendency to move around. In A
Study in Scarlet he mentions that he was hit in the shoulder, while
in The Sign of Four he is experiencing pain from his war wound in one
of his legs. In The Adventure of the Noble Bachelor a wound is
mentioned in one of the limbs, but we don't know which.

This is of course cirumstantial evidence, but it seems that we are
not told the whole truth about Watson. Maybe he has an evil twin who
sometimes manages to upstage him?"

Interesting Eh?

Manraviel






--- In sherlockholmessocietyofindia@..., "Ashoke
Dasgupta" <ashoke_dasgupta@h...> wrote:
>
> If she called him James instead of John in "Man with the Twisted
> Lip," was it a Freudian slip indicating she'd someone else on her
> mind?
>
> Seriously, that was an odd mistake for Doyle to make.