There are two varieties of sirloin steak available, both from the sirloin primal, which is sandwiched between the short loin primal (behind the ribs) and round primal (the rump). Think mid-back area. The steak that Alton gushed over comes from the top sirloin, a section of meat found under the tenderloin. The bottom sirloin is right under the top sirloin, and is a much tougher cut of meat with little in the way of fat. Most butchers like to use the bottom sirloin for ground meat, or cut it into chunks for stewing or braising.
Chef's Resources - Top-Sirloin