Which actually makes perfect sense: if the ebook came out too far ahead of the first print edition, it would hurt sales of the latter too much.
I normally do a similar thing with the newsletter I work on: even though it's certainly possible to send out the PDF edition in advance (and much easier now that the group has a couple of list servers set up), and with this month's edition, time constraints made it necessary to send out the PDF to all members, including those who normally get only the print edition, before the print edition was even ready to go in the mail, I normally don't even generate the PDF until at least a few hours after the print edition is in the mail, because I don't want to penalize those who choose to receive only the print edition.