Some people just make it look easy. How is it that certain players have the speed, physique, and strength and others just don't have such luck? Is it nature versus nurture? I would say NATURE.
I was always amazed at the ability of some players to come into the Woody Hayes with a bag of McDonald's and a Coke. They would throw that down right before physical activity, and they would perform like a champion on the field. Some players didn't work out as hard as others in the weight room, but come "40 yard dash" time or "game" time, they would perform far superior when compared to everyone else. The gene code was just written different for some.
You can improve strength and technique, but I could never squat what Andy Katzenmoyer or AJ Hawk could squat. You can improve a vertical leap, but all the plyometrics in the world won't get me to Ahmed Plummer or Dustin Fox range. I believe that is determined well before people get to Ohio State. Terry Glenn may have been a late-bloomer, but it isn't a case of "Terry all the sudden got confidence in what he is capable of achieving." Make no mistake about it, Terry Glenn could run faster than the defensive backs that were on him.
I love the Spielman story. "Chris is just a normal guy that had a big heart," people said. "He is the ultimate over-achiever." Work ethic and intangibles may have taken him from good to great, but he can physically do things many, many other linebackers can't do. He could run a forty. He could position his body better than others. He bent his knees and played at different pad levels. He was strong enough to take on offensive guards and super-sized fullbacks. There isn't anything "just a normal guy" about him. He was physically gifted.
Draft day and letter-of-intent-day are less art and more science than many of us wish for. There are plenty that have been "nurtured" very well. However, the ones that "nature" took care of are gobbled up first by the coaches.