Hanteo basically has relationships to fanbases, for example Chinese fan bars. They will count orders from those bars as albums sold even if they aren't printed yet.
Gaon Charts are literally produced by the Korea Music Content Association and sponsored by South Korea's Ministry of Culture, Sports and Tourism. They are the most accurate measurement for sales we have. Any ecxess albums will be returned at the end of the year and subtracted from the total.
Hanteo on the other hand is relatively unreliable as they only use their verified stores and also double count albums that are returned and bought again. It is extremely Korean and Chinese heavy, especially Chinese buyers basically all buy from Hanteo certified stores (especially Ktown4u) with their mass buying.
It's basically always Hanteo heavy fandoms that try to discredit Gaon because they want to make their groups look better.
