The problem is that the grand total line is calculating the sum for the rows, including the sum of your avg4 column. The following might not be exactly what you are looking for, but hopefully puts you in the right direction:

CLOSE TABLES all
 
 CREATE TABLE sampledata (party_name c(20), prod_name c(20), yearname n(4), sal_qty n(6))
 
 INSERT INTO sampledata (party_name,prod_name,yearname,sal_qty) VALUES ('PARTY A','PRODUCT A',2010,45)
 INSERT INTO sampledata (party_name,prod_name,yearname,sal_qty) VALUES ('PARTY A','PRODUCT B',2010,40)
 INSERT INTO sampledata (party_name,prod_name,yearname,sal_qty) VALUES ('PARTY A','PRODUCT A',2011,30)
 INSERT INTO sampledata (party_name,prod_name,yearname,sal_qty) VALUES ('PARTY A','PRODUCT B',2011,25)
 USE
 
 DATAPATH=SYS(5)+CURDIR()
 oExcel = createobject("excel.application")
 oExcel.Application.Visible = .T.
 oWorkbook = oExcel.Workbooks.Add()
 oTargetSheet = oWorkbook.Sheets.Add()
 oTargetRange = oTargetSheet.range("A2")
 oPivotCache = oWorkbook.PivotCaches.Add( 2 ) && external data
 oPivotCache.Connection = "OLEDB;Provider=vfpoledb.1;data source=" + DATAPATH
 oPivotCache.Commandtext = "select * from sampledata"
 oPivotTable = oPivotCache.CreatePivotTable( oTargetRange, "PivotTable" )
 oPivotTable.PivotFields("party_name").orientation = 1 && row
 oPivotTable.PivotFields("prod_name").orientation = 1 && row
 oPivotTable.PivotFields("yearname").orientation = 2 && column
 oPivotTable.PivotFields("sal_qty").orientation = 4 && data
 
 * Calculate % change
 oChangeField=oPivotTable.AddDataField(oPivotTable.PivotFields("sal_qty"), "avg4")
 oChangeField.Calculation= 3  && xlPercentOf
 oChangeField.BaseField="yearname"
 oChangeField.BaseItem="(previous)"