AggregateFunction in SSAS is not working good for me

I have a field on which im using SUM aggregate function in my SSAS project. data in that field is never negative. but when i roll up i get negative values. when i drill down data looks accurate. any idea what is happening. here is the metadata
        <Measure dwd:design-time-name="5efdf859-7977-4f21-83ef-3b37f2067b9f">
          <ID>Delta Msec</ID>
          <Name>PLT1-ApP</Name>
          <Source dwd:design-time-name="f6b3921a-402f-4950-aa93-a8a264709a78">
            <DataType>Integer</DataType>
            <Source xsi:type="ColumnBinding" dwd:design-time-name="9acf4d7c-8e77-4a1a-ba84-eb5931af9725">
              <TableID>dbo_Data</TableID>
              <ColumnID>delta_msec</ColumnID>
            </Source>
          </Source>
          <FormatString>Standard</FormatString>
        </Measure>

strange thing here is when i set aggregate function to null. i get
<AggregateFunction>None</AggregateFunction> in the metadata.
but when i change it to sum i dont see <AggregateFunction>Sum</AggregateFunction>

Am i doing anything working. why am i getting negative values in SUM