Why system can deduct amount when setup Withholding Percentage, but cannot deduct when setup as With

Hi,

In project billing, I input Project>Billing Information> Retention>
add "Withholding Term" > "Withholding Percentage", the
Generate Draft Invoice. The amount of retention is correct.

But when I input Project>Billing Information> Retention> add "Withholding
Term" > "Withholding Amount", then Generate Draft Invoice.
The draft invoice did not deduction the retention amount.

Kindly advise, why system can deduct amount when setup Withholding
Percentage, but cannot deduct when setup as Withholding Amount?

Thanks a lot

Phasiri