% Chance to increase potential tier?

Facepalm, occam's razor. Unnecessarily complicated and would give exactly the same result in the long run. Why randomize twice if your first randomizer is already good enough?
The "random" part of it is like getting a random number from 1 to 100, the set chance is that it works if that number is lower than 10.

Because if someone figured out the pattern for the first number, the second number helps guarantee that it can't be abused as easily.

Assuming this is how it works though. I and i hope others like this theory of "how scrolling works" the best.