I can write off that the nu-Mirror-universe Enterprise looks like the 1701-A due to it being an alternate timeline. And I can easily disregard that the nu-1701 looks like it as an error.
It would make sense if the ENT mirror design is different-looking. If I remember right, the ENT-D from TNG: Dark Mirror was actually supposed to resemble a Sovereign-class. (Albeit a LOT bigger!)