Yes, I thought it was odd that they should title it "Master and Commander: The Far Side of the World" as though there would be a whole "Master and Commander" series. Since he has the rank of Master and Commander for only two of the books, and then toward the end of the second is rated Post Captain, a rank he maintains until book 20 (barring the occasional temporary commodore's rating), it would be more logical to call them "Post Captain" and set the films during that phase of his career. I can only assume that "Master and Commander" sounded more marketable.