This is because the recipient has already fixed the amount to be paid as this is a Dynamic QR Code. Hence, when you scan the DuitNow QR code, you will be making a payment for the exact amount. The amount will be displayed on the screen for your confirmation.

For more information on the type of payments, the differences are as follows:

STATIC QR CODE
QR code displayed by participating merchant or recipient which requires you
to key-in the amount of the payment or credit transfer.

DYNAMIC QR CODE
QR code that is generated after the merchant or recipient keys-in the amount
of the payment or credit transfer. You are not required to key-in the amount
for this scenario.