This is a question that many have asked in the past, amigo.
No one is really sure of the true answer; but, here is what I have to offer (my conclusions are drawn from years of casual research and discussions with various Italian Western aficionados and filmmakers)....
The artwork for the poster is actually based on an American Western pulp paperback cover (one of the many books from the Buchanan series by Jonas Ward). I used to have a copy of this paperback---but, haven't been able to locate it for the past year, or so. The Italian artist took the exact same painting from the aforementioned book cover and merely put Garko's head on the original figure and recolored a few things.
But, as you have pointed out....the figure bears zero resemblance to how Sartana appears in the film. So, it seems that this was one of the many "pre-release" posters that were done in those days. Usually with these advance posters, the artist simply had no clue about the film's actual content. He figured..it is just another Western so any Western looking art is OK (remember the pre-release A FISTFUL OF DOLLARS posters with the generic cowboy gunfighting scene?).
The second release poster is much, much better..showing Garko with the rifle over his shoulder as seen in the opening sequence to the film.
I have an original Italian poster for the second release (it is HUGE) and I have a French poster with the very same artwork, as seen below.