(This is ALL magazines, not just the flying ones)
It's called "competition".
Whatever day of the month you bring out your comic, sooner or later your competitor is going to bring his out a few days earlier, so as to appear on the bookstalls first, so that the punters who want to buy one comic on this topic but not all of them will buy his new comic first and then not buy yours as well when it comes out a few days later.
So, next month you publish a few days earlier, don't you, so that you can be first.
It is easy to see that after a few years of this you get January numbers published in mid-November.