Different method for reporting 53 weeks vs 52 weeks. Chartz used a 53 week year, but Famitsu used a 52 week year last year. Because of this, New Years is in 2020 for Chartz and in 2021 for Famitsu.
This will make the comparison difficult because we will be comparing a 53 week 2020 with a 52 week 2021. Week 1 in 2020 ended Jan 4, while this year it ended Jan 9. This means that most holiday weeks will not match up year to year.
Not sure the best way to handle this as there will probably be wild swings in comparisons some weeks, but in the end we'll still have a full-year comparison either way.