In a nutshell.
The rank shown on each issue is the rank of the chapter shown 8 chapters ago. Exceptions are of course new series ( less than 8 chapters), unless they really, really do bad on the preliminary results, series with less than 8 chapters are just put on random spots, though its likely they get a color page in chapter 3, 4 or 5.
This is almost confirmed because almost each time a great or particular chapter appears, 8 weeks later that manga ranks well.
I find comeplling evidence in hokuto no ken, said by bakuman to be the 1st chapter with most votes ever IIRC around 800. 8 weeks later (chapter 9) ranks 1st 2nd ifbyou count color pages).
Alsonyeah, Oda did say so himself.. that's how they do it.
Other exceptions are pimped mangas that get a bazillion color pages, Toiko at its early months, Bleach before falling a lot (IIRC the last couple of volumes with espadas?), and with the popularity ofseasonal anime of jump properties i.e if Kuroko gets a season 2 it will get lots of CP.