I can only say that it was probably about a month after I started with Netflix that they sent me a mail like that, asking if I was getting the DVDs in one day. I don't think it has anything to do with 'optimizing their shipping' at all. How long have you been with them? The last 2 weeks (after basically 2 months with them, with about 20-22 dvds/mo), they've 'delayed' each shipment at least 1 day and then sent me dvds from as far away as they could find (one from Tacoma WA, another from Lexington KY), that were both avaiable here in TX. So, I don't know what to tell you, although my first instinct b/c of my experience would be to ignore/keep deleting the emails, as that might be their 'first step' to 'slow you down' to 12/mo. and get that data in their system to use basically against you.
Others have said their system uses algorithms for shipping quotas and such, and it seems true. I would just keep ignoring the emails asking when you get the dvds, because they'll try to use it against you in the end, to minimize the DVDs you recieve and to maximize their own profits.