Per the Farmers Almanac
The harvest moon can come in September or October because it is the full moon closest to the autumn equinox.
2013 that happened with the September full moon so the October full moon is called hunters moon.
In years where the harvest moon comes in October the September moon is called the Corn Moon.
So basically we have 3 names for 2 moon cycles (corn harvest hunters) Depending on when the autumn equinox occurs in relation to the full moon we use 2 of the names. Either corn and harvest OR harvest and hunters.
What makes these two moon cycles different than the others is the moon rises closer to sun set. On a normal month the moon rises about 1 hour after sunset. On these two months the moon rises about 30 minuets after sunset. This may also occur around the spring equinox but I did not see anything about it in the almanac.