What cause such creasing on my shirt?
Do you have some notification system that alerts you when new members post questions or do you just constantly refresh in order to be one of the first responders? You should be commended for the lengths you go in order to respond to every one of these new member questions with something that is completely unhelpful, not amusing, insulting or a combination of all three. Your dedication and devotion to this calling is unprecedented and amazing.
I measure a shirt that I consider fit well for me when I create my custom shirt size, so I suppose this shirt will turn out well. But numbers that they require doesn't include rounded armhole length indeed, and I choose double extra super slim when i place my order, that may cause the problem. I measure my MTM shirt after reading starro's reply, and find out my new shirt's rounded armhole length is at least half an inch shorter than the old one.
His shoulders don't look that sloping to me. I think the culprit lies in the slim fit. In today's fashion climate, a slim fit usually means they skimp on material in many places. So while it may fit on you stationary, unsightly lines appear when youre in any sort of motion.
One possibility: not enough fabric in the back.
As to measurements,
1. Realize it's MTM
2. Hard to tell from picture what posture OP is in.