Hi Andryis,
This is due to the functionality of Line Numbers and Release Numbers. You will notice that every PO line has both these numbers.
Line Number remains the same for a Part on the Purchase Order. The release numbers are supposed to represent different deliveries (based on the concept of delivery drop - different Wanted Delivery Dates). The system groups the same item and displays them together using this concept.
In my experience if there is a requirement to view or print the Purchase Order Lines in a sequence, then a custom field could be used to define a basic sequence. The Purchase Order Lines can then be sorted on that basis, and same can be used in updated Purchase Order report.
Hope this helps.
Regards.