Here's why:

* Lunar Cycle: The lunar cycle, from new moon to full moon and back again, takes approximately 29.5 days.

* Calendar Months: Most calendar months have either 30 or 31 days. This means that a full moon won't always occur within the same calendar month.

* Occasionally Two Full Moons: Sometimes, due to the slight difference in length between the lunar cycle and calendar months, a month might have two full moons. This second full moon is called a "blue moon".

So, while a full moon occurs roughly every month, it doesn't happen precisely every month.