Here's why:

* Lunar Cycle: The moon orbits the Earth in about 29.5 days, which is called a synodic month.

* Lunar Phases: As the moon orbits, the amount of its sunlit side we see changes, creating the lunar phases. A full moon occurs when the entire sunlit side of the moon faces Earth.

* Variations: While a full moon occurs roughly every 29.5 days, the actual timing can vary slightly due to the elliptical shape of the moon's orbit.

So, while it's not exactly once a month, it's a pretty good approximation!