The order that we see is a direct result of specific choices made when the purchase is generated.  

In The Financial Edge if you go through Records, Requisitions, and select the Generate Purchase Orders option when selecting a requisition, you notice different options for grouping multiple requisitions together.

A Requisition in WebPurchasing should have the same line item order on the Requisition in The Financial Edge. After this, the system goes by the generate purchase order selections:

Examples:
Generate Purchase Orders from Requisitions:
How do you want to group requisition line items?
By Vendor Only

OR

By Vendor and purchase order information such as submitted by, deliver to, buyer, department, etc.
How do you want to create PO line items for the same product?, etc.

If the created requisition contains all of the same vendor only, and the choice is made to put into 1 Purchase Order only, then the line item order should mirror the requisition line item order.  Otherwise, the system is going to split the line items into separate POs, thus breaking up the original requisition line item groupings. 

At the point where the Purchase Orders are generated, the system will show you a preview of what the new assigned PO line item number will be (should we be choosing to split the 1 requisition into multiple Purchase Orders.)