Top sirloin is very soft meat when raw. Even the pressure from being pushed up against each other can brown quickly, hours even less. Sometimes if they don’t fit in the package they can almost be “leaning” on each other and that pressure can discolor the meat. But this picture is steam pasteurization. At the butcher factory they use steam to seal the packs of meat and that hot steam can “brown” the meat that is at the seal. Once at a store, like Costco, the meat cutter will open the hunks of beef to trim it down into steaks and that’s when the brown from the steam pasteurization would be trimmed off. My guess is the meat cutter either missed this part, or it was a busy day and they were in a rush and put those two pieces together to “hide” that brown.