How exactly is wastewater usage calculated?

Trying to figure out our bills since recently moved here. For September and October, our water usage was 2 CFF, the “Wastewater Service” section of the bill reads:

Wastewater Usage Charge $ 92.55 King County Treatment Charge $ 110.22 Wastewater Taxes $ 11.12 Total Wastewater Service $ 213.89 

For November and December, our water usage was 5 CFF, the “Wastewater Service” section of the bill reads:

Wastewater Usage Charge $ 18.51 King County Treatment Charge $ 110.22 Wastewater Taxes $ 7.06 Total Wastewater Service $ 135.79 

Why are the months with less water usage charged with much higher “Wastewater Usage Charge”?