You have probably checked the flag "fixed quantity" for the PO item (it's the last field of the item line)
Delivery Date and Quantity Fixed
Indicates whether you want to fix the delivery dates and quantities that
are confirmed after an availability check.
Use
If the supplying plant cannot confirm the desired delivery date or the
desired delivery quantity in total, confirmed dates and quantities
differ from the desired dates and quantities. With this checkbox you can
control whether materials planning is based on the confirmation schedule
lines or on the desired schedule lines.
o If you select the checkbox, the goods are made available on the date
confirmed by the availability check in the supplying plant. The
system only generates confirmation schedule lines for the
requirement quantities confirmed by the ATP check.In the case of
underconfirmation, desired and confirmed quantities deviate from one
another. You cannot reschedule the corresponding purchase order
item.
o If you do not select the checkbox, materials planning is performed
according to your Customizing settings. In the Customizing activity
Assign Delivery Type and Checking Rule, you can use the ATP Conf.
MRP field to indicate whether materials planning is based on desired
or confirmed schedule lines. You do this for a combination of
document type, supplying plant, and checking rules. If you select
the checkbox in Customizing, confirmation schedule lines are still
taken into account in the case of rescheduling.
Note
If you select this checkbox in a stock transport order, the system
ignores the Customizing settings described above for all schedule lines
of the purchase order item in question.
