Hi Niaz,
an important first step in analyzing supposedly wrong result sets is to avoid comparing apples and oranges.
As Excel can only consume the analytic view but not pure SQL it doesn't help too much to compare the Excel result with the plain SQL result.
Instead, it would be more interesting to see the results of a SQL statement that queries the analytic view.
Unfortunately the data preview function does create a nested multi-grouped statement (you might check the statement via the 
"View log" button on the upper right corner of the data preview view) so that results are not necessarily comparable to those of your Excel report.
Therefore: what's the result of
SELECT
"SalesTerritoryGroup",
SUM ("ResellerOrderQuantity")
FROM "_SYS_BIC"."adventureworks/AN_SELLERSALES"
GROUP BY "SalesTerritoryGroup";?
- Lars