SqlDataSource SelectCommand, Parameter Order Matters?

Hi all,

I've been trying to rack my brains over this one; When I use this select statement in SqlDataAdapter, it runs fine; it takes about 4 to 5 seconds to run.  In SqlDataSource, its very erratic; taking between 8 to 71 seconds to run.  I've finally narrowed down the issue to something that resolves the symptoms to something in my query;

SELECT     f.UnitID, f.FacilityName, SUM(CASE WHEN t .sData_Type = 'CLINICAL' THEN t .nWrite_Off_Amt ELSE 0 END) AS Sum_Clinical,
                      SUM(CASE WHEN t .sData_Type = 'OTHERS' THEN t .nWrite_Off_Amt ELSE 0 END) AS Sum_Others, SUM(t.nWrite_Off_Amt) AS Sum_Total,
                      SUM(t.nCashAmount) AS Sum_Cash, t.iYear
FROM         t LEFT OUTER JOIN
                      d ON t.sUnitID = d.sUnitID AND t.sPat_Num = d.sPat_Num AND t.iMonth = d.iMonth AND t.iYear = d.iYear LEFT OUTER JOIN
                      tblFac AS f ON t.sUnitID = f.UnitID_1
WHERE     (t.iYear = @year) AND (t.iMonth = @month) AND (@UnitID IN ('08948', RIGHT('0' + f.Division, 5)))
GROUP BY f.UnitID, t.iYear, f.FacilityName, f.Division


If I change my WHERE clause to the following

WHERE     (@year = t.iYear) AND (@month = t.iMonth) AND (@UnitID IN ('08948', RIGHT('0' + f.Division, 5)))


performance is MUCH better.  How could such a little change make such a big difference in speed?

Thanks.

-e