I have no idea of the real answer to your question, but, when early releases of Windows 8 were available, Microsoft themselves, boasted at great lengh on the future "Store". It was stated that all applications would be vetted before being plced in the store, in order to maintain a standard.
My own observation, in the latter stages of Windows 8 development is that 1. The Store has now become so large, that it is quicker to "google" for anything that you seek, rather then plough through it. ". There does not appear to beany standard at all being applied. Merely in order to comment and criticise, I have installed and uninstalled many apps across the board, to examine what is available. Many of these are written by codeing amateurs, who have use the well advertised, and comaparitively simple, method, of turning a small application, or simple game, into a full blown Metro. The result is a prolification of , in 60% of what is available, total garbage. No standard is being applied. The available stars, to which you refer, do not, in many cases, relate to the quality of the program.
So, as in my first sentence, I am not aware of any "formula" being applied. I seriously doubt that any group in Microsoft is dedicated to going through every offering and assessing each.