Requisition changes the price of the item when sent to the Shopping Cart

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Hi, 

I have the following question:

1) Requisitions> Enter Requisition Lines
2) I enter the item number
3) Price = 0.21 (unit price of the master item)
4) Add to shopping cart

5) click on Review Shop Cart
6) The price of the item has changed as we have a Blanket Purchase Agreement for that item

I researched several posts and understood that this is how this scenario is correct

But, I would like to understand the reason for the scenario below:
1) Requisitions> Enter Requisition Lines
2) I enter the item number
3) Price = 0.21 (unit price of the master item)
4) I change this value to 1.00

5) Return the price to 0.21