Negative Unused Space

I wrote simple script to check space used by tables:

CREATE TABLE #SpaceUsed(
TableName NVARCHAR(128),
NoOfRows INT,
Reserved NVARCHAR(18),
Data NVARCHAR(18),
Index_Size NVARCHAR(18),
Unused NVARCHAR(18)
)
GO
sp_msforeachtable "INSERT INTO #SpaceUsed EXEC sp_spaceused '?'"

SELECT * FROM #SpaceUsed

SELECT
CAST(Sum(CAST(Replace(Reserved,' KB','') AS INT)) AS NVARCHAR) + ' KB' AS TotalReserved,
CAST(Sum(CAST(Replace(Data,' KB','') AS INT)) AS NVARCHAR) + ' KB' AS TotalData,
CAST(Sum(CAST(Replace(Index_Size,' KB','') AS INT)) AS NVARCHAR) + ' KB' AS TotalIndex_Size,
CAST(Sum(CAST(Replace(Unused,' KB','') AS INT)) AS NVARCHAR) + ' KB' AS TotalUnused
FROM #SpaceUsed

DROP TABLE #SpaceUsed


and one of results looks strange to me:

TableName NoOfRows Reserved Data Index_Size Unused
-------------------------------------------------------------------------------------------------------------------------------- ----------- ------------------ ------------------ ------------------ ------------------
T_TableXX 50081 38024 KB 37432 KB 640 KB -48 KB



Anyone know reason of such result (negative value of unused space)?