Thanks – yeah looks like you had different talents selected, now I see the same thing. It probably only happens with Seraphim builds.
I have a couple tweaks that should make it not do that as often, but the short version of why it is doing that: using the Mythic+ simulation and your talents and gear, replacing some of your mastery with critical strike simulates to just about the same value.
I would have to do some analysis to give you a more in-depth answer to why that is (other than “the data says so”), but off the top of my head: crit really isn’t a bad toughness stat for protection paladins, particularly with Seraphim where you are hurting for SotR charges. More crit helps with that. There’s probably some point that the simulator is finding where getting a bit of crit helps out more than stacking extra mastery for toughness.
Given access to any gear in the game (e.g. running Best in Slot), it will probably go more Mastery in an all-toughness build. But limited to only what you have in your bags… sometimes there is a “local” solution that is of similar or higher value. The optimizer is good at finding these “unusual” or non-obvious gear combos.