The legal argument is very simple - if you go off to war as a serving soldier the chances of sustaining a permanent injury are pretty high, so the compensation is correspondingly low. When you tip up at the typing pool at 9 o'clock every morning you are not expecting to be the victim of a life changing injury, so when you can "prove" you have one - Ker-ching!
We all know it's bollocks, but that's the marvellous justice system we have.