Maybe the one he used couldn't show that the balance was 800k for the first 3 months of the previous extension and never fell below 400k for the rest of the 12 months before reverting to 800k
Correct, there is a 60 day "Visit Thai Family" extension but some Immigration Offices will not issue this on a Visa Exempt or Tourist Visa entry stamp so the OP needs to visit Immigration to see if it's possible