It appears the number I pulled was counting the current definition of blue moon and the old definition of the third full moon in a season of four full moons. Which apparently was very important for the old lunar Christian ecclesiastical calendar. Thirteen moons a year would throw it all out so ignoring one of the moons as a 'Blue Moon' kept it all together.
You learn something every day, even if it is of no real use, then again the same could be said of my uni lecture today. 'Dredging hydraulic transport technology.' Less exciting than it sounds trust me
