Sterling will, and does, score plenty of tap-ins. That appears to be his job. City attack an area just to the edge of the 6 yard box and then have 3 options; shoot, pull it back to the edge of the box of cross it to the far post. Sterling's job appears to be getting to the back post.
It's not rocket science, just be in the place where Pep has told you to be. They create so many chances, if you're in the right position you're bound to score quite a few. Even then, he has missed quite a few sitters this year.