Does sp_spaceused data contain more than the sum of each table data?
What I understand is that the sum of the tables will not be equals the database size. On msdn it says:


  database_size will always be larger than the sum of reserved +
  unallocated space because it includes the size of log files, but
  reserved and unallocated_space consider only data pages.
  
  Pages that are used by XML indexes and full-text indexes are included
  in index_size for both result sets. When objname is specified, the
  pages for the XML indexes and full-text indexes for the object are
  also counted in the total reserved and index_size results.
  
  If space usage is calculated for a database or an object that has a
  spatial index, the space-size columns, such as database_size,
  reserved, and index_size, include the size of the spatial index.


Reference here