Revision of PO is creating on Save Button

Dear All Experts,

We are facing an issue that when user is going to change a approved PO it goes to approval requires status, but whenever lines are adding or deleting it creates a revision of PO without sending to any approval.

And in approval history it shows as user send it for approval when deleting or adding each line, but in actual user is just adding and deleting the lines. Why this behavior is being shows. Any help will be appreciable.

Thanks in Advance