they only take in acount your "muscle mass" by measuring your waist, thats it. its not the most acurate of tests, i was 16 stone and 6'2 and i got 29.5 and got failed temporarily, the limit is 29, im now 14.9 bmi 26.... but im big built and heavy, not fat, and i still failed, and if your 5'11-6ft at 15 you might be on the limit.
Again, a recurring theme. The issue guys, is that you do not want to bulk-up your muscle mass too much before recruit training because the excessive weight you carry, albeit not fat, will make running at speed over the distance unnecessarily arduous during recruit training
The important thing to remember is if you pass medical you may also pass PRMC but it's important to balance CV exercise & upper body strength in equal measure.