Sum of table sizes don't match with mdf size
There are "internal" tables, queues, etc that don't show up in certain reports / queries as they are system objects. If you run the query below, which counts the number of actual pages defined in the data files in the current Database, you should see that one of the "Reserved" fields (match the appropriate "KB", "MB", or "GB" field to your report) matches the "Space Used" value in the Data File portion of that report.

;WITH cte AS
(
  SELECT COUNT(*) AS [Reserved],
         SUM(CASE WHEN pa.[is_allocated] = 1 THEN 1 ELSE 0 END) AS [Allocated]
  FROM sys.dm_db_database_page_allocations(DB_ID(), NULL, NULL, NULL, DEFAULT) pa
)
SELECT  [Reserved] AS [ReservedPages],
        [Allocated] AS [AllocatedPages],
        '---' AS [---],
        ([Reserved] * 8) AS [ReservedKB],
        ([Allocated] * 8) AS [AllocatedKB],
        '---' AS [---],
        ([Reserved] * 8) / 1024.0 AS [ReservedMB],
        ([Allocated] * 8) / 1024.0 AS [AllocatedMB],
        '---' AS [---],
        ([Reserved] * 8) / 1024.0 / 1024.0 AS [ReservedGB],
        ([Allocated] * 8) / 1024.0 / 1024.0 AS [AllocatedGB]
FROM    cte;


And below is the query to get the per-object details along with a grand total line at the top:

SELECT sch.[name], obj.[name], ISNULL(obj.[type_desc], N'TOTAL:') AS [type_desc],
       COUNT(*) AS [ReservedPages],
       (COUNT(*) * 8) AS [ReservedKB],
       (COUNT(*) * 8) / 1024.0 AS [ReservedMB],
       (COUNT(*) * 8) / 1024.0 / 1024.0 AS [ReservedGB]
FROM sys.dm_db_database_page_allocations(DB_ID(), NULL, NULL, NULL, DEFAULT) pa
INNER JOIN sys.all_objects obj
        ON obj.[object_id] = pa.[object_id]
INNER JOIN sys.schemas sch
        ON sch.[schema_id] = obj.[schema_id]
GROUP BY GROUPING SETS ((sch.[name], obj.[name], obj.[type_desc]), ())
ORDER BY [ReservedPages] DESC;