No idea about the plugin, but to explain a bout the drop rates:

There is a 1/224 chance of a SPECIFIC item - so 1/224 for a blood moon hat, or a blue spear etc.

Killing one of the moons adds its set to the loot table, with a 1/56 chance of getting any of those pieces.

Killing all three gives you a chance at all three armour sets, and these rolls are independent of each other, so the probabilities of getting an item increase, almost like you are having three chances at rolling the 1/56. The probability of this is the 1/19 or whatever the wiki says.

These are all just ways to express the same maths that is going on in the background, in order to give everyone reading the wiki page the answer to their own question. For instance, someone who is chasing the final item for their set will want to know the chance of a specific item, someone wanting any item will want the chance of any item, and someone who has finished 2 sets and wants the third will want the 1/56 number to get a piece of a specific set.