How do these different Django 'context' construction differ?

Question

The basic concept of context in Django are well explained here and here, as well as Django doc. But I don't understand the following code that works in the Mozilla Django tutorial:

class BookListView(generic.ListView):
    model = Book

def get_context_data(self, **kwargs):
    context = super(BookListView, self).get_context_data(**kwargs)
    context['author_books'] = Book.objects.filter(author=self.kwargs['pk'])
    return context

Why is it necessary to define context twice? Is it the same as: author_books = # 'author_books' defined book_list = # 'book_list' defined

    context ={
        'author_books': author_books,
        'book_list': book_list,
    }

Testing in Django shell didn't work for me as I'm still struggling with how to extract data from foreign-key items.