Here is the snapshot ID for my frost mage: 143bd3b24702472da92af6a9bf798144
I have noticed lately that when selecting best in bags it wants to really bring my crit rating down to pump in more mastery. Since mastery tends to be the worst stat I was starting to wonder why it wanted to drop my best stat for my worst stat. I ran a sim using what BiB is suggesting along with what was recommended based on frost discord, raidbots, and general tutorials on what to look for to beef up my mage. The sim is showing that the BiB is roughly 3k short of the top setup which is in my eyes a big difference. I do apologize if this is somewhat confusing since im still relatively new to using AMR to its fullest. Hopefully the screenshots will help explain.
Here is the best in bags
Here is sim results