What you don't understand that the top corner in the draft probably isn't the first taken?
It does not always happen the consensus "best" corner really is. That is why the position is very much like WR, hard to evaluate due to systemic biases and translation to the NFL. Plus they are often flaky and it takes the right mental makeup to get it in the NFL.
2015 - Trae Waynes, Kevin Johnson, Marcus Peters - we know who the best one was, Waynes has been close to bust level
2014- Marcus Gilbert, Will Fuller, Darqueze Dennard - none of them really; best corner is Bradley Roby who was picked 31st
2013 - Dee Milliner, D.J. Hayden, Desmond Trufant - Trufant, hands down, Xavier Rhodes was taken next and might be even better and Darius Slay went 36th
2012 - Claiborne, Stephen Gilmore, Dre Kirkpatrick - Claiborne was a bust, fourth picked was Janoris Jenkins
2011 - Patrick Peterson, Prince Amukamara, Jimmy Smith - Peterson obviously.