This is really a calendar issue and has already been cross-posted in AstroTorah.
However, it does involve some statistics so it is applicable here, as well:
We just read the הפטרה of פרשת פינחס which is a pretty rare occurrence. However, it is not the most rare. The exact frequencies of each rare הפטרה are discussed here. The numbers are based on a chart of the frequency of each type of year found here.
However, there is one issue with the calculations. It assumes that whenever אחרי מות and קדושים are separate and קדושים is not a special פרשה, its הפטרה is read. However, as pointed out in R' Dovid Heber's שערי זמנים, this is not the custom, according to משנה ברורה. Rather, as long as the הפטרה of אחרי מות was not read for whatever reason, we read that in place of הפטרת קדושים. Therefore, the הפטרה of קדושים is only read when both אחרי מות and קדושים are not special פרשיות, thereby dropping the frequency under 6%! For more thorough details please see this excerpt from the ספר.