The data on the right shows which stats to go for, i.e. “generally speaking, getting gear with thoes stats simulate better than your current gear”.
The BiS function only uses gear you currently own and gives advice for that specific situation.
Gearing currently includes so many different factors that it’s not possible to give general advice like “haste is always better than crit” or “never use versatility” as there’s far too much interaction between stats. While there may be an optimal stat distribution that doesn’t necessarily mean other options are bad.
You can click on the “View optimal stat graph” on the right to get an overview of lots of different stat distributions with tooltips showing how big the difference would be.
If I take a look at the current recommendations for my blood dk, there’s less than 1% simulated difference between having 3000 and 800 mastery. As I’m unable to reach 3k mastery, gearing for another value gets recommended but the data on the right will show “you may want to get more mastery”.