Answer
CO2 might be low because more plants are making food and due to mineralization
Step-by-step 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.