Apparently, the discrepancy comes as a result of time. Troops whose wounds are discovered after they return home are not added to the official tally.

Rep. Bill Pascrell (D-N.J.), founder of the Congressional Brain Injury Task Force, said the total number of Americans who may have suffered head injuries in combat maybe total as many as 150,000 — about 10% of the 1.5 million troops have served in Iraq

“I am wary that the number of brain-injured troops far exceeds the total number reported injured,” he says.

surprise, surprise --NOT

Psychiatric injury, like 'battle fatigue' or 'shell shock,'can be ignored, because there may be no clear physical evidence . These can be some of the most devastating injuries, because physical injuries can be understood. Psychiatric illness in general is still subject to a state of denial.
Particularly the psychiatric ramifications, of being sent away as heroes, and returning to a society that has no memory of the send off when they return. After serving they return to neglect, and denial. Often told in the 'land of the free.'
War Wounds? Sorry we just pack 'em up and ship 'em out. When they get back they're free to fix their own problems, from scratch.