I totally understand where you are coming from.
In my case I think it's because the PTSD didn't fully surface until years later after I suffered an additional trauma. Some of the original trauma never fully surfaced and some of the memory isn't even there for me to access and process. It doesn't feel real.
Just remember that PTSD can't be faked. If you were diagnosed it is likely that the clinician saw a lot of things that can't be faked. When I was diagnosed I didn't even know what the problem was, I just thought I was going crazy.