Stahp. That's how it works for every book, CD, greeting card, and assault rifle manufactured around the world. Somewhere in Japan, at some point, some day there will be a bookstore that needs to restock One Piece and they are not going to want to wait for a reprint. Shueisha hopes to have those extra copies waiting in a warehouse somewhere, ready to load on a truck, to deliver as soon as the need arises. The same applies to every book they publish. They print more than they think will sell so that later they have enough if a retailer runs out. Sometimes they run out of a title and need a reprint, it can't always be helped.To be honest, I do not see why they are printing 4 million for OP again. The previous few volumes barely made it above 3 million even after many months (talking about the top selling volumes list of 2013). Not that 3 million is not a huge ass number in itself or anything.
Oh, I guess that means One Piece's 18 million+ copies sold in 2013 means nothing then. Thanks for clarifying.

---------- Post added at 11:38 PM ---------- Previous post was at 11:26 PM ----------
The international market is very different than the Japanese market. In the US Bleach also has always been as well known as One Piece and Naruto. Five years ago it was better known than One Piece. The anime used to air on Adult Swim, a popular night time animation block. Bleach aired with popular non-anime like Family Guy (Adult Swim revived the series after it was originally canceled by Fox) and Futurama. Beyond sales I don't know very much about how Bleach was received in Japan. I know that the anime market was very different when Bleach was at it's peak. Everything I have seen suggests that now Bleach is a title in decline. It's decline in sales and TOC rankings suggest to me that casual Japanese fans have mostly abandoned the series. It's been left with enough fans to still sell well, even after a sizable decline. losing 200 - 300,000 copies per volume in sales could not have made Shueisha happy.My explanation was probably poor so you misunderstood but I didn't mean sales by reach, but only the people that know of the series. I was only talking about what I experienced in France (2nd paper market in manga after Japan), where Bleach is as well known as OP and Naruto for example. In Japan I assume this is basically the same, Bleach is really well known and I think it's reach is more important than HxH simply like you said because the age targeted isn't the same. I think Bleach targets like you said too early teen while HxH older teen between 14-16. Or for example Kuroko no Basuke has a really important reach towards female readers.
And while sales is a poor measure of reach and overall popularity I think it's still an indicator of the popularity of the manga.
It is very hard to find demographics for individual manga. A few years ago NHK conducted a poll to see who is buying One Piece. It turns out that 88% are 19 or older. I would expect Hunter x Hunter to be about the same. Bleach I don't know. The anime was marketed at that 12 - 15 range, but that does not necessarily apply to the manga. In it's day Bleach was a lot like Kurobas or Haikyuu, very popular with women.
