I dunno know.
I figured with more water to preside over as the earth warms and the glaciers melt (ala al gore), there would be more pirates to take advantage of the newfound floating communities.
I was thinking that data were real (possibly)....but there are no standard weather records kept before 1870 or so....thus the 1860 and 1820 numbers are guessed at. I'm not sure about the number of pirates....it's not unreasonable to think that the maximum was in the mid-1800's....