The sirloin is actually divided into several types of steak. The top sirloin is the most prized of these. The bottom sirloin is less tender, much larger, and is typically what is offered when one just buys sirloin steaks from a supermarket. However you get a bargain price with Sirloin steak too. 

With all this said, we at Jim's Meat Market trim the fat close and cut the bone /grizzle off. So you get only the leanest portion possible. This makes for the tenderness cut of sirloin at a reasonable price.  Little things like this will keep you coming back.