So the damage absorbed/reduced is less, but you can do it more often. I'm not an expert but that doesn't sound like a pure nerf to me.
Disc is dead anyway in pvp. Yaaay, so many changes to the spec. Rightful punisment for first month of expac overperformance, I guess.
Blizz just simply can't into fixing disc priest for pvp
Its likely a change for PvE reasons, because Disc bubble would be brokenly OP for raiding if someone had the PvP trait essence.