I see one, even two clear reasons for the company's silence.
They have always made black hole-like secrets out of their business details - the evil competitors are right at their door steps!
And, tiering up, as well as levelling up, is easy-peasy in the beginning but it becomes harder at each step. It's better for them to maintain the mist of secrecy on these subjects so that their clients do not see the massive grind evolving to move up. And, our grind is money for our hosts!
Transparency is why I gave so much energy to the reference tables for the levels and was always interested in the tiering details. Alas, these have become too complicated by now.
This is not to say that it is illegitimate, but it's a business model nonetheless, and successful!