I can't say for sure, but to me it looks like a muscular imbalance. Ever muscle has an antagonist. And they should be about equal in strength.
Have you ever seen the guys who bench all the time, but neglect most of the other muscles, but especially their back? Their shoulders are forward, and hands are turned in when standing relaxed. Thats because their chest and front delts are naturally pulling more than their rear delts, lats, rhomboids, ect.
Do deads, rows, and pull ups or pull downs. Working the muscles that pull your shoulder blades together will make them stay together more naturally. And even if you have a slight alteration in your bone structure, its obvious that it is nowhere near chronic, so adding musculature to your back would make it impossible to see IMO. Good luck!