Perhaps the extra 20% is in anticipation of an incoming Mastery nerf? I read somewhere that mastery stacking leads to some pretty insane PvP burst. I have no PvP experience myself, so take that with a grain of salt.
it isnt much of a concern in pvp, particularly arenas, since you dont start at 10 stacks of tigereye brew. you need to spend 30 chi before you can even get to that point, and mastery stacking has no benefit till you use brew.
I think the devs realized the 10% stance nerf was too much, and even if it was reverted, ww monks would still lag far behind the likes of rogues/warriors/dks.