Jason is completely right! It's caused by the way dependency injection works. It's based on hierarchical injectors.
There are several injectors within an Angular2 application:
When Angular2 tries to inject something in the component constructor:
So if you want to have a singleton for the whole application, you need to have the provider defined either at the level of the root injector or the application component injector.
But Angular2 will look at the injector tree from the bottom. This means that the provider at the lowest level will be used and the scope of the associated instance will be this level.
See this question for more details: