Hanteo and Gaon count albums differently. Hanteo doesn't include digital albums I think. And TVXQ albums always been high. They don't need to sajaegi.
More like:
Gaon tracks "how much album was shipped - the ones returned" on a monthly basis and Hanteo only tracks like 70~80% of all the Korean music retails stores and they use some formula to make up for the 20~30% they can't track and make an estimate.
Neither tracks digital albums cuz that's not a thing in Korea ( ) and in this situation, TVXQ probably didn't sell as much as SM had anticipated