Evaluating players for the NFL when they're still in college is a lot like buying a futures stock. Every player is an individual commodity, and right now is when the value of each can fluctuate. And just like stocks, there is risk with every decision, but using the information available throughout the season, you can make informed decisions on the performance and potential of each. And that's what drives up a player's stock or causes it to crash.
It's becoming trendy in draft circles to say that "draft stock" doesn't exist. It does. How else would you explain the rise of Carson Wentz last fall?