If the promotion occurred before 1864, your relative may have been what was known as a 'Yellow Admiral' - a Post-Captain who had reached the top of the seniority list (and therefore was next in line for promotion under the rigid seniority rules), but who was not considered suitable for an executive position; such officers were automatically promoted, but then immediately placed on the retired list to make way for a more junior Post-Captain to take the promotion (to the rank of Rear Admiral of the Blue Squadron) into an active appointment.