There is no set stat priority. Many different combinations of stats will yield optimal or near-optimal results. We stopped using simple stat priorities and stat weights a long time ago. Our gearing strategies are based off of millions of simulations we have already done. The optimizer finds the best setup that you have the gear for and puts you as close to that as possible. In short: we pick the gear we do because that is what simulates best for you.
Mastery loses a lot of value in BiS situations with the 8.2 patch, so it is not surprising that BiS has lower mastery. It actually becomes the worst stat if you are using certain essences. On a spread cleave fight it would still be one of your best stats, but not single target.
You can use the “customize” tab on BiB to push the optimizer towards a particular stat or trait. We are working on adding more customization options for those who want particular stats.