Chulasakarat era date conversion (for Thai coins)

Hi mates, I see a wrong date conversion for Chulasakarat calendar on Numista.
Well-known resource Creounity Time Machine reports the conversion rule for Chulasakarat calendar as
Year AD = Year CS + 638. For instance, 1238 CS = (1238 + 638) AD = 1876 AD. But on Numista CS 1238 converts as 1877, so extra year is added. Can any body check it?
Regards

It's not a solar calendar so neither is always correct they start their year on the 13th April.

Surely if the bulk of the year matches when adding or subtracting 638 this is the best conversion? Why prefer a conversion that works for 3 months of each year to one that works for 9 months?