Are pages allocated on extends, however this pages are not used yet by any kind of objects. It means, SQL Server has 8 pages by each extent. If an object reside in a page of a certain extend, this space is allocated and used, but the rest of the space of the extend is allocated but unused because there are not objects.
Space (extends) that is not in use, it means extends that are not allocated by SQL Server. Any kind of grow will produce a space unallocated.