[AccessD] Calculation in query problem

I have the following query
SELECT [tbl Produced Gas Meters].Meter_ID, scada_V_All_Meter_Volumes.ReadingDate, [Volume]*1000 AS Mcf, Sum([Volume])*1000 AS McfTest
FROM (([tbl Produced Gas Meters] 
INNER JOIN scada_Gas_Meter_Master ON [tbl Produced Gas Meters].Meter_ID = scada_Gas_Meter_Master.Meter_ID) 
INNER JOIN ([qry Last Gas Analysis Date] 
INNER JOIN (GA_Header INNER JOIN GA_Details ON 
GA_Header.ID = GA_Details.HEADER) ON 
([qry Last Gas Analysis Date].MaxOfSample_Date = GA_Header.Sample_Date) 
AND ([qry Last Gas Analysis Date].METER = GA_Header.METER)) 
ON [tbl Produced Gas Meters].[Meter_ID Text] = GA_Header.METER) 
INNER JOIN scada_V_All_Meter_Volumes ON scada_Gas_Meter_Master.Tagname = scada_V_All_Meter_Volumes.Tagname
WHERE (((GA_Details.UNIT)="PCT"))
GROUP BY [tbl Produced Gas Meters].Meter_ID, scada_V_All_Meter_Volumes.ReadingDate, [Volume]*1000
HAVING ((([tbl Produced Gas Meters].Meter_ID)=362915) 
AND ((scada_V_All_Meter_Volumes.ReadingDate)=#12/1/2011#))
ORDER BY scada_V_All_Meter_Volumes.ReadingDate;

The expression [Volume]*1000 AS Mcf returns the value 18259.5043182373

The expression Sum([Volume])*1000 as McfTest returns 200854.54750061

I have a feeling I am missing something in the order of calculations as done by Access. 

Any thoughts appreciated.

Chester Kaup
Engineering Technician
Kinder Morgan CO2 Company, LLP
Office (432) 688-3797
FAX (432) 688-3799

 
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