Here's why:

* Lunar cycle: The moon's cycle from new moon to full moon and back again takes about 29.5 days.

* Calendar months: Calendar months vary in length (28 to 31 days).

* Timing: Since the lunar cycle and calendar months are not perfectly aligned, sometimes a month will have two full moons, and sometimes it will have none.

However, the average frequency of full moons in a month is close to once.