Ah! The history of periodical publishing.

The practice goes back to the early days of news stand distribution. And probably greatly influenced by the daily newspapers.

The news stand operators would let a publication stay on the stand up to the end of the date on the cover. So, of course, the dailies were cleared off the stand at the end of the day.

Publishers of monthly titles wanted to have their magazines on the stand as long as possible. So they would put a month "date" on the magazine that was actually 1 or 2 months after the publication date. That meant that something labled "November" that came out sometime in September would have at the very least 4 full weeks on the stand, and possibly 7 or 8.

It's all about sales.