by Celedam » Sun Oct 30, 2016 4:03 pm
Some of it is due to the weekly period that each chart or program uses: from Monday to Monday, or from Friday to Friday, or whatever. Where the release falls during that week can greatly affect the ranking, because it determines the number of days that are actually counted.
It's more so for second- and third-tier idol groups that are in the rankings for only one week anyhow, before they're pushed out by more mainstream artists that can stay in the rankings for several weeks or even months.