What kind of meat did I buy from Costco?

Wife and I were perusing the meat department yesterday at our local Costco for any mislabeled packages. We do this just for the fun of it when we go. We came across the package of Shabu Shabu meat and it had really nice marbling on it. It was listed as sirloin on the wrapping, but we were wondering if by chance it is a different cut of meat entirely because of the marbling? Any opinions? BTW, it is currently being marinated in Yoshida sauce, from Costco of course.