Why do I get an "Access Level must be protected or weaker" after extending a protected class variable and marking it private?

abstract class AbstractController
{
    protected $repository;
}

class GraphController extends AbstractController
{
    private $repository;
}

I get this error:

Fatal error: Access level to GraphController::$repository must be protected or weaker

Why? What's the theory behind this? On some level it feels wrong that I could have a weaker access level to a class property (i.e. public) when I extend a class, because I am in a way exposing a variable that was meant by parent class to be more restricted...