by Celedam » Wed May 17, 2017 1:07 am
Different periods, probably. Oricon is Monday-to-Monday, and it looks like Billboard is Sunday-to-Sunday. So that's one extra day of sales going into the Oricon numbers, at least in the first week. And for a release like this (i.e., an album, a singles collection, only one edition), the usual idol sales tactics aren't inflating the Billboard numbers.
It's the same issue as with Music Station, which is Friday-to-Friday if I remember correctly. Until we figured that out, people were going nuts that MM would get #1 on Oricon but #4 (or whatever) on Music Station.