I believe his total service in Nam was longer than that. Considering the "seriousness" of his wounds, I'd say his petition to leave was somewhat legalistic. I knew lots of guys -- enlisted and draftees -- who would have jumped at the chance. Officers, however, would generally prefer to stay where the action is.
For example, LBJ's son-in-law was assigned to Cam Rhan Bay, but there was no action there. For whatever reason, no doubt political, he asked to be reassigned. Ambitious people do not generally look for easy ways out.