When you shop for a new car, you really aren't sure of what you are getting until you take it out for a test spin, look under the hood and kick the tires a bit.
Think of the NFL combine as an auto auction of sorts. You don't know exactly how a player will perform until you have a chance to see them lift, run sprints, catch, throw and block.
And just like buying a car, what looks good to some potential teams may not look so good to others. With so many choices to make a decision from, they need to see more success and evaluate performance based on their own criteria.
Welcome to the NFL world of scouting.
While there are players who have entrenched themselves as "certain" top-10 selections, there are still questions that remain about players and their ability. Are they sliding down the draft board? Why are they considered high risk? How will they perform at the combine?
These players are looking rise up the draft board after they have had a chance to show NFL scouts what they can do on the field and what they can do in big-game situations.