8818 is the rivet average in the current wr game, which is largely a result of six sigma rng.
RNG is random number generator, which is where randomness comes from.
Six sigma means that something was 6 standard deviations above the mean one way or the other, in this case on the good side. In a normal distribution this is well within the top 1% of potential outcomes.
Standard deviation is the square root of the variance.
Variance is the sum of all squared differences between the mean and actual results. For example, in the set 4,8 the mean is 6 and the variance is (4+4)/2, so 4, the standard deviation is the square root of 4 so 2. You square it to kill negatives, basically.