The temperature of the combustion air affects its density and, therefore, the actual mass of oxygen passing through the burner.
Cold air
It is denser and contains more air mass in the same volume.
Warm air
It is less dense, so the same volumetric flow carries a smaller mass of oxygen.
Seasonal effect
A burner adjusted at a very different ambient temperature may exhibit small variations in O₂ and CO values.
Best practice
Combustion measurements should also record the air temperature so that comparisons are meaningful.