Hi Bill,
The approval is generally on full amount probably because of below reasons
1) Keep it bit easy for approvers otherwise they would have to be difficult to identify whether they are approving whole order amount or it is just an additional amount. For instance, as in your example, original order is $40,000 which is already approved, now for approval of second version of the order, there might be chances that approver didn't noticed that the 25k is the additional amount and approve it assuming it's total order amount. Advisable is to go call approval of the whole order everytime.
2) There might be other fields which are validated in the custom approval rules which might be based on commodity codes, amount etc. Hence they will also need to be taken care of whether they should be triggered only on additional amount or whole amount.
Advisable is to validate whole order amount instead of only additional amount.
Amit