Actually it's no mystery.

by R. Proffitt Moderator - 11/9/12 9:00 AM

In Reply to: Update for today by AllOtherUsernamesTaken

MAC addresses have been spotted to be the same many times. Here's something about it.
http://forum.xda-developers.com/showthread.php?t=1385577

So it's not something in the hardware at all and as you discovered, not a reliable way to block devices.
Bob