In the big year of 2021 and still people don’t know the difference between Hanteo and Gaon and using the gap as a drag. The first time I saw that if I’m not wrong was with BTS back in 2017 with LY: Her and now in 2021 with NCT 127 STICKER sales.
BTS is the artist with the biggest gap between those 2 charts and the reason is super simple: they have their albums being sold in non Hanteo certified stores because HYBE and ARMYs prioritize Official local charting in each country.
The same thing is happening with NCT 127 and the same happen and will happen with other artists. For others like BTS for example the gap will be big because they are the biggest korean artist internationally and now NCT 127 after their exponential growth internationally.
You just have to take a look at STICKER debuts on tons of Official charts around the world. Sometimes you have to buy those albums in Hanteo non certified stores in order to get them to chart. The ideal thing would be having those sales to count on both but not everything is perfect.