I have a friend who crushes me in any sprint. He can be completely untrained and just blow me away on a 400m at a moment's notice. These days, he trains 5k and the mile. He asked me to run with him today, and I know without a shadow of a doubt keeping up with him will be a piece of cake. Hell, I could beat him on 3 mi runs when he was training regularly and I was an anorexic skeleton that couldnt beat an old lady up a flight of stairs.
One day when he was still training for track meets, I joined him for a session of five 400m reps. Between reps, we walked 400m to recover. On the first rep, he absolutely mauled me, probably finished a whole 20s earlier. But the very next rep, I beat him by 5 seconds. By the fourth, I was beating him by 10+ seconds. On the last rep, he rallied hard and I still cruised past him down the final leg, crossing the finish 20-30 m ahead.
If I never trained with this guy, I couldn't believe people varied so much. He never caught his breath during the whole workout, even though we walked for MINUTES between each rep. Just recently, we sprinted up a hill after going on a walk, probably 200m at 5% grade. He killed me, of course, but I caught my breath almost instantly and carried on conversation fluidly as we walked back to his house, probably a quarter mile away. By the time we got to his house, he was STILL breathing hard. We sat down and continued our conversation. For the next five minutes, I kid you not, his speech was broken up by big breaths, and he could only take small sips of his drink.
Similarly, he's very strong on squats, almost reaching a 2x bodyweight squat with minimal training. I only broke 2x bw one time, barely, and I trained like my life depended on it. On the flip side, I've done 135 for over 100 reps before, and I'd be willing to bet he could never even get 50.
The point is, I've seen how this guy trains -- I actually taught him how to train in the beginning -- and the intrinsic differences between us were immediately apparnet. I could train purely anaerobic while he trained purely aerobic, and we would still beat each other in the contests were intrinsically suited for. The gap between us is just beyond the limits of adaptation. And if we train for our respective strengths? The other can just forget about it; he has no chance.