First of all, you should avoid using integers to store dates, even if they are just partial as month and year. You get in lots of trouble when calculating change of years (in this case).
Here's a possible solution but something else might be better.
--Parameters
DECLARE @Year int = 2013,
@Month int = 2;
--Variables for previous dates
DECLARE@PrevYear int,
@PrevMonth int;
SELECT @PrevYear = YEAR( DATEADD( MONTH, @Month - 2, CAST( CAST( @Year AS CHAR(4)) AS datetime))),
@PrevMonth = MONTH( DATEADD( MONTH, @Month - 2, CAST( CAST( @Year AS CHAR(4)) AS datetime)))
SELECT @Year AS [Year],
@Month AS [Month],
MAX( CASE WHEN [Month] = @Month AND [Description] = 'Stores' THEN Value END) AS [SumStores],
MAX( CASE WHEN [Month] = @Month AND [Description] = 'Stores' THEN Value END) -
MAX( CASE WHEN [Month] = @PrevMonth AND [Description] = 'Stores' THEN Value END) AS [Diff],
MAX( CASE WHEN [Month] = @Month AND [Description] = 'SalesStores' THEN Value END) AS [SumStores],
MAX( CASE WHEN [Month] = @Month AND [Description] = 'SalesStores' THEN Value END) -
MAX( CASE WHEN [Month] = @PrevMonth AND [Description] = 'SalesStores' THEN Value END) AS [Diff],
MAX( CASE WHEN [Month] = @Month AND [Description] = 'Products' THEN Value END) AS [SumStores],
MAX( CASE WHEN [Month] = @Month AND [Description] = 'Products' THEN Value END) -
MAX( CASE WHEN [Month] = @PrevMonth AND [Description] = 'Products' THEN Value END) AS [Diff]
FROM (
-- this selects a list of all possible dates.
SELECT [Year],
[Month],
SUM(Stores) Value,
'Stores' Description
FROM #ABC
where ([Year] = @Year and [Month] = @Month)
OR ([Year] = @PrevYear and [Month] = @PrevMonth)
GROUP BY [Year], [Month]
UNION
SELECT [Year],
[Month],
SUM(SalesStores),
'SalesStores' Description
FROM #DEF
where ([Year] = @Year and [Month] = @Month)
OR ([Year] = @PrevYear and [Month] = @PrevMonth)
GROUP BY [Year], [Month]
UNION
SELECT [Year],
[Month],
SUM(Products),
'Products' Description
FROM #GHI
where ([Year] = @Year and [Month] = @Month)
OR ([Year] = @PrevYear and [Month] = @PrevMonth)
GROUP BY [Year], [Month]) AS T
;