% Chance to increase potential tier?

You realize you did the same thing right?

How do you know it's a set chance? What if it's a base chance that is modified by a multiplier randomized by the server for an undetermined period of time? Huh?

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.