Sorry if I was insulting your intelligence with things you already knew. I just took your question at face value and assumed you were asking why muscle tightness occurred.

The answer to your question is that medical science doesn't know why some people end up with conditions like TOS and others don't. I have extremely tight shortened muscles with all the risk factors plus I am a manual wheelchair user so should be a prime candidate for TOS but I don't have it. I do however, have a rare incurable neurological condition, the cause and mechanism of which is even more of a mystery to medical science than TOS. The inability to determine why some people get a disease or medical condition and others don't is a universal problem throughout the whole of medicine.

If medical science actually knew what causes some people to develop illnesses, diseases and conditions and others not to, then it would know how to cure them. With a fairly small number of exceptions, it doesn't. All medicine can do for the vast majority of conditions is treat symptoms.

I don't think looking in the mirror is necessarily a very reliable way of assessing the detailed features of underlying structure, positioning and health of internal soft tissue and bone. You may look less uneven than you used to but there is clearly still a difference between your right and left side and still an underlying problem which you would be unlikely to see for yourself by looking in a mirror. Add to that the fact that nobody has a symmetrical body even if it 'looks' much the same on both sides. Sometimes tiny defects or differences make all the difference.