What's the reason some of the Detective Neuro's manga cover's page are upside down?

Question

I have 12 out of 23 volumes of Majin Tantei Nogami Neuro. It's really weird since the first tenth volume of this manga's cover is upside down like you could see in this site.

What's the reason behind it?

Answer 1

There's no particular reason other than "because the mangaka wanted it that way".

It's just one of the many omake/additional fun things about the tankobon (according to Japanese Wikipedia):

• The characters on the cover page are upside-down on vol. 1-10, as-is on vol. 11-20, and left-turned on vol. 21-22(23)

Some other trivia:

• The name of the criminal of that volume

  is hidden in the background of wall-of-text in the table of contents (except vol. 15, 16, and 23), e.g. vol. 11's criminal name. (Source)

• The illustration on the middle cover of vol. 10-19

  uses "Hanging Yako" representing "1", e.g. vol. 18. (Source)

• The cover pages for vol. 13-20

  are connected: 13-16 and 17-20. (Source)