I posted what may be a partial explanantion of some of these issues in another thread (my post here: https://www.videohelp.com/forum/viewtopic.php?p=1427929#1427929)

The gist of it is;

From Netflix help:
In determining priority for shipping and inventory allocation, we give priority to those members who receive the fewest DVDs through our service. As a result, those members who receive the most movies may experience that (i) the shipment of their next available DVDs occurs at least one business day following return of their previously viewed movie, (ii) delivery takes longer, as the shipments may not be processed from their local distribution center and (iii) they receive movies lower in their Queue more often than our other members. By prioritizing this way, we help assure a balanced experience for all our members. Those that rent a lot of movies get a great value and those with lighter viewing habits are able to count on our service to meet their limited needs.