In the days before computers, this would be an incredible amount of work to keep track of what came in when.
And it was important to keep track of that because most periodicals were sold to retailers on a returnable basis.
(This got out of hand eventually, and you'd have comics' publication dates off by 6-8 months!
That is, if the retailer didn't sell everything he ordered in a given timeframe, he could return them to his distributor for a refund.
With magazines and comics, though, publishers frequently tried to look more current than they actually were.So they'd print a date somewhat later than when the book actually went on sale; a book that was actually published in January would have a February date and would (theoretically) look more current than the other magazines next to it with the actual date.