I'm not quite sure what you are trying to get at here. Adding 1% incline to your run requires an extra energy output of 4%. 2 degrees is 8%.
The maximum run time for a young chap is 11.13. Adding a 2% incline to this would require you to able to run the mile and a half in 10.30 on the flat. Just by adding a 2% incline you add 43 seconds to the time of your run. I assume that the navy picked the values because they know something about how fit they need their recruits to be.
Another point to note is that level treadmill running uses next to no upper leg muscles. Adding incline makes the upper legs muscles work harder. If you aren't used to these muscles working in that way you would need more training to meet the times mentioned above.
Don't get me wrong, I think I agree with what you are trying to say to some extent. A 10:30 mile and a half is quite easy, however, why would someone worry about passing the PJFT if they could only run it in 11.12?