You are assigning the SelectedValue as -1 in the below code, due to which it is assigning the default value, which is zero:
cmbDelayedDeliveryDays.DataSource = m_dtDelayedDeliveryDays
cmbDelayedDeliveryDays.DisplayMember = "DeliveryDelayDays"
cmbDelayedDeliveryDays.ValueMember = "DeliveryDelayDaysID"
cmbDelayedDeliveryDays.SelectedIndex = -1 //Comment this line and check again.
Hope this solve your issue...
If any queries do let me know.