Answer
CO2 might be low because more plants are making food and due to mineralization
Explanation
Plants absorb carbon dioxide to make their food (energy) and in turn give us oxygen, in the process called photosynthesis, therefore, during that season, there is a chance that more plants were planted to reduce the CO2.
Some minerals naturally react with CO2, turning carbon from a gas into a solid, this is called mineralization, this could also have been the reason for CO2 reduction.
