User blog comment:IamJakuhoRaikoben/Chapter 381: Where Demons Live, Review./@comment-86.203.165.55-20140424080855/@comment-7418318-20140424135635

That's how I understand the matter: Instead of leaving out an issue number on a break, the Japanese magazines put both numbers onto the issue before or after the holiday, purely for organisatorial reasons. This practise at some point coined the term "double issue" without there being anything double in it, except for the naming of the magazine issue itself.