It is "more or less" correct that the main beach plus the beach at Bloody Bay add up to the old British unit of measure (miles) that was still being used at the time the poster was made. The metric system had not yet been adopted. Thus the nickname from that era and it has stuck as referring to the main beach.
But the term "seven mile beach" was derived incorrectly from the poster that read "Seven Miles OF Beach in Negril', which included the small beaches on the West End.
