A lot of OP volume purchases are front loaded. It's weird. Volume 101 has 1.5 million copies sold which is higher than most manga volumes in December save for Jujutsu Kaisen, but it sells very little beyond that point and it drops. Seriously go check oricon weekly. OP is sells extremely well then on following week's it just drops off.
Oricon weekly December 27th-Jan 3rd it was 8th, the following week it was 37th, but the volume in total still sold more than most other manga on that list save for Jujutsu Kaisen
Like you have something like new prince of tennis barely beating one piece vol 101 with 33K copies, but it only sold 33K in its first week of release, while one piece sold 1.5 million with its volume, but only 32K that week.
Hopefully that makes sense.