Randomness is random, yo. I only glanced it, but sanahtlig(sp?) made that thread the other day to try and calculate the likelihood of particular rolls, but I assume he posted the numbers and it ain't pretty bro.
Some days I'm just trying for a particular group of SRs or any SR or even Rs but all of the same element, and the gacha just trolls me with Amon+Tyr rolls that I don't need instead. It's a lifestyle choice, escape while you can.