I think maybe there is a bit of tension/just a bit of shortness in the back/binding, but oh my goodness, with it so slight in this case, it isn't anything I would bother with myself.
If it is going to be on a wall and that corner is going to be right at your eye and if it really really bugs you, you might try taking out that last 12-18" of the back side of the binding and see if you can't adjust it back out. If that doesn't do it, next step would be to completely remove the binding about 1 foot from either side of the corner and try resetting. Sometimes though our fixes create bigger problems and I would count it as sealed and done!