This is simply raw data on the cost of a pirate versus the amount of DEF she provides. Skills related to DEF have been noted. Pirates are listed in order of best to worst for convenience.
To explain, the ratio is how much DEF you get per 1 point occupied. This does not necessarily mean that Pirates at the top of the chart are your best bet or the bottom of the chart are your worst choice. Total Defense power, skills and the total amount of points you have to spend are more critical deciding factors. This data is to show how much "bang for the buck" you get for your pirates.