Of course we should not expect all afflicted veterans to get over their PTSD on some given schedule. The fact that some people get over their problem relatively quickly does not in any way mean that all those who are affected should be on the same timetable. Different people have different levels of PTSD. They have not all been subjected to the same levels of stress. In addition, people’s minds are different and different people can be affected more or less by the same experiences. Different people have different social situations in the present. The differences in things like support systems surely cause different people to heal at different rates.
For all of these reasons and more, there is no reason to expect that all veterans who suffer from PTSD will recover at the same rate.