Still my imagination says, It's possible.
I would ask you to check these two sales order. I bet there is difference in the requirement type of these two line items.
You can go to procurement tab in sales order and check for requirement type for these two SO.
Next task would be to see the strategy in SPRO (Availability check and transfer of requirement) and check its corresponding requirement class and type. And I hope the configuration of reqt type and class will explain the reason behind non-peg orders.