If everything were super clean, it would be possible to determine if the heat capacity used in the calculations was for 0 C, 20 C, or 40 C and if the air was supposed to be dry or humid.
The heat capacity of air hardly changes at the extremes of humidity. The humidity in a closed room in Sapporo does not change measurably from day to day. This experiment is not being conducted in a jungle.
Such things cannot explain a 60% change. I am sure you know that. Stop grasping at straws.