I wonder why they do 3+mln print runs for this yet theres according to this, still over 850K copies just sitting out there. meanwhile, they seem fine doing numerous small print runs for heroaca and other series, constantly leading to, (well according to iceemperor) constant stock issues.
At 60K a month, it would take 15 months for them to rid themselves of the remaining one piece copies.
Oricon are not covering 100% of the sales, only about 40,000 shops around Japan, and there are far more shops selling manga that are not int the stats. Convenience store such as 7-Eleven, Lawson, FamilyMart and others... every store have a few copies of the latest volume, there are 60,000+ stores like that in Japan. Adding library, archives in Shuiesha and others, manga museums like the one in Kyoto and Ishinomaki, every editor getting his copy, doing the math and all the 3+ million copies are spread out.
Not sure when but as soon as the copies stars to run out they print a second edition, back in April I found volume 86 2nd print in 7-eleven near my hotel.