Acquiring initial grip on the pistol is usually more complicated when done from concealment, rather than from an open carry holster. A poor initial grip could result in less accurate hits, even if they are delivered with the same speed.
If I'm not mistaken, this is why the FAST drill requires the first shot from the holster to be on the index card: to penalize shooters who just make a sloppy grab for the pistol on a quickdraw.
I think Caleb's point is very well made: are people just comparing their best draw speed from open vs concealed? Is first-hit accuracy being compared? Are the number of fumbled draws being compared?