Pivot Table Excel Grand Total Shows the Sum of Unformulated Calculated Field Value

Question

I tried a simple data table,

Table 1

ITEM	TARGET
A	150
B	150

Table 2

ITEM	PROD_QTY
A	160
B	120

I append both of the tables and create a Pivot Table, then use calculated field to see the outstanding by using these formula:

=IF(TARGET-PROD_QTY<=0;0;TARGET-PROD_QTY)

It worked well with the sum of outstanding, but when I was see the grand total, it shows like this:

ITEM	SUM OF OUTSTANDING
A	0
B	30
Grand Total	20

Is there anything that I could do to fix the grand total? Cause I think the Grand Total is ignoring the =IF function in Calculated field and just go straight to the calculation