Planned Purchase Order does not require update of amount limit on terms page

We have a purchase order for $487k with the terms limit of $499k. The PO amount was changed to $550k, over our 3% reapproval limit. The header amount changed and the PO was sent for reapproval where it self approved - did not go on the route. When checking the limit of $499k did not change on the terms page. When we changed this the PO did require reapproval. Why didn't the header amount change force a change on the amount limit on the terms page.