Well even before the fun started I knew what BEagle was on about and I've never been in the forces as such.
The police service had a similar line of thinking. They would deduct your normal mileage in the principle that you would have used it to go to work anyway. So I go to station x, distance 10 miles each way. My normal run is 5 miles each way. Home to x would therefore allow me to claim 10 mile for a return run but if I had gone to work booked on and then gone to station x I could claim 20 miles and overtime if it was outside my rostered hours.
I presume is that similar to what BEagle had.