Perhaps someone mentioned it and I missed it, but IIRC there are two versions of 321 NEO, one with 8 doors, and one with 6 doors and 4 overwing window exits.

From the photos it appears that UA has taken the latter version, so there is no need to have the lost real estate in the exit doors before the wing, allowing it more seats than AA or DL.

Again, I am going on my memory as I have a hard time actually searching this on the net