Best in Bag is bar to use for multiple specs

I do understand what it’s trying to do. For casual readers of this post it’s trying to help you stat your best gear for each spec min/maxing the overall stats for each spec in the priority order that you have chosen (left list on the screen).
My issue is that I delete all but my main spec form the list, run gear check and it suggests more haste. When I equip that gear and run gear check it suggests replacing all the haste stat gems/ enchants with more mastery. For my class I understand mastery for mythic+ is superior so I tend to believe gear check.
Gear Check says:
Gear better than yours often has:
Less Crit
Less Haste
More Mastery
Less Vers

I also don’t understand the recopmmendation to use a one hander as main weapon when I have no offhand available at all. One hander is 395, two available staffs are 375 and 380.
ilvl 395 with 69 haste, 58 mastery vs ilvl 380 135 crit, 1050 int, 105 mastery, 1393 int., I guess the crit is just worthless in the sims overall. It just seems off.

If you post the string of data that you copy from the addon to the website, we can try to reproduce your case and see if anything is amiss.