BEING A CREATURE OF HABIT, I ALMOST ALWAYS HOLD my iPhone in a vertical orientation when making pictures. That's cuz, since I primarily make square pictures, it doesn't matter which orientation I use. So, I use the orientation that I normally use when holding / viewing my iPhone.
Now that I am frequently playing with the Portrait setting for it narrow-er DOF quality, + the fact that there is no square setting-although I can crop to square in processing-in the Portrait Mode, + the fact of my use of (habitual) vertical iPhone orientation, all of the recent full frame pictures I have made are in the vertical format. This relationship of habitual practices and their result just dawned on me. Duh and more duh.
Time to break old habits and hold my iPhone in a horizontal orientation.