Amount in approval proccessing.

Hello, I have an issue and hope you can help me

- I've created a non stock purchase order with two lines, $300 and $500 respectively.

- The order went through an approval process and was approved.

- I received the fisrt line.

- Then I changed  the amount of second line to $1000.

- The order went back to approval process with an order amount of $1300, amount commited $1000 and requires approval to the level up to $1300.

Why is the system considering the amount of closed line ($300)? It is working as designed?

Regards.