Another factor in this: out of all positions in basketball, I think centers tend to have the most parity amongst projection out of high school and performance in the NCAA.
One example: look at the top centers in the '05 247 rankings: Skal Labissiere, Diamond Stone, Steven Zimmermann, Caleb Swanigan, Chance Comanche, Daniel Giddens, Tyler Davis. Out of that group, Swanigan is the only guy who had a truly impressive college career.
That same year you have guys like Fall, Carvacho, Alexander, Koumadje found much later in the rankings.