Remember when you are racing a two hour harescramble, there is no breaks between quarters or time outs. You are giving it your all the entire time or you shouldn't be out there.
Before my first race (the Moose Run, Morrison, IL, six hour race) I had been running and lifting on a regular basis. At one point in the race, I was on my hands and knees trying to get my legs to uncramp so I could get back on the bike and keep going. I have always been a good athlete and in good physical conditioning. The rigors of racing totally blew my mind.
Remember they aren't just analyzing your shear muscle, how much you can lift, your endurance, lung capacity, cardiovascular system and whatever else you can think of, but they are analyzing the whole package. Sure, the top football players and hockey players are in great shape, but the demands of those sports are a lot different than MX or Offroad racing. They are obviously more muscular and bulky, but does that truly make them in better overall condition. As mentioned earlier, soccer is the number one rated sport for physical conditioning. If you look closely, you'll find they compare more evenly to a top dirt bikers physique.
Like MX221 said, I am sure they are looking at the top athletes in the sports.
Not that I am biased or anything.
Neal