So... If none of you know anything about statistics and probability, a RNG is a random number generator. There are several ways to implement this in terms of resource generations. Here are a couple.
(a) Every single block, roll a "dice". Depending on the value of the dice, set block value to "x", where x is the block ID for whatever is rolled. There are more faces to this die that are stone, than there are of ores, obviously. You roll a different dice depending on what y-value you are at, as copper doesn't spawn below "n" and diamond only spawns below "m".
(b) Generate an string of numbers, similar to a SEED. i.e. The number used to generate your world. That algorithm may contain several implementations that define a systematic way to which your world is randomly generated. i.e. Put swamp next to desert next to ocean, repeat three times then start next world gen algorithm. This algorithm would also contain the RNG that determines your ore generation.
In the end, the mod can only control the probability that an ore spawns. In order to determine whether an ore gen is OP you would need to take several samples of multiple independent chunks, even worlds and determine the mean value, standard deviation and other statistical values. Once those are known you can state the ore gen rate as x +/- y per chunk and compare it to (a) other ore gen statistics or (b) previous values of same ore gen, prior to mod addition.
I have 15 minutes till the bus comes, give me a break. I'm bored.