It's generally less that people think that you'll be able to do what you did last time again, and more that people believe that you're the kind of person who delivers on their promises. If you want to get a promotion at work to best place to start might be doing something as simple as taking everyone's lunch orders and getting them right. The point isn't to show that you're really talented in one specific area, but rather to create an asset, that asset being a group of people who trust you'll deliver on your promises.