This practise is a remnant of the printing times. Papers would get accepted and then printed in a later issue. But once the online publishing started, this kind of was not necessary anymore. Which lead to online publication before print, but somehow still using the print date for the article because a lot of journals still have physical prints.

That said, I don't know if this journal does that and then if not it is simply stupid. They might do it because they limit "online" issues in size, like the printed ones. Which is idiotic if you don't actually print anything.