This topside of beef cut comes from the inner muscle of the thigh. This muscle is quite tender and very lean so it makes an excellent roasting joint. Your butcher will often secure a layer of fat to the top of the to baste it as it roasts, which ensures your end result is succulent and delicious. You can just pop that off the top of your roasting joint after cooking if you prefer not to eat it.
Also referred to as the beef 'Rump', the beef round primal cut basically consists of the back leg of the steer. Muscles from the round are fairly lean, but they're also tough because the leg and rump get a lot of exercise. Just like the sirloin primal is separated into two subprimals, top sirloin and bottom sirloin, beef round likewise consists of multiple subprimal cuts: the top round (inside round), bottom round (outside round), and the knuckle. The bottom round is where we get rump roast and eye of round.