water, As I said, I specialize in posture (I am a chiropractor) and based on the frontal view, I guessed that you had a "forward" shoulder. I guessed this because your left shoulder looks closer to your body than your right. Most americans slouch, so I made an assumption that the shoulder was forward and not back. I suggested that a side picture would help, which would have cleared up my mistake. Based on your experiment I have the following advice... Don't modify your posture to accomodate shirts that are made for slouchy americans. It turns out that your poor shirt fit is from GOOD posture.
I suggest ordering any future Jantzen shirts with the "square" shoulder option based on your experiment. BTW congrats on having decent posture, it truly is among the most important aspects of good health. If anyone is curious, check out Ideal Spine