The most important difference between the major and the other wounded men is that the major is older than the other men. This is true both chronologically (we assume) and mentally or emotionally.
We can know that the major is older partly because he is a major -- a rather higher rank than the others have and therefore probably older. We can also see it from the fact that the narrator refers to the others as "boys" who are the same age as he is. He does not refer to the major in that way.
The major is also at a different level than the boys in terms of his mental state. He is much more serious and mature than they are. He has been a champion fencer. He is married. He does not go hang out with the younger men after they have worked on the machines.
The major is on a different plane than the others in terms of his physical age and his mental/emotional seriousness and maturity. This is one reason why he is the one whose circumstances end up being the most tragic -- he is the one who would feel things most profoundly because of his mental and physical age.