Total emissions aren't the issue here - it's the emissions of specific regulated compounds like NOx. Most emissions from engines are in the form of CO2, which

Total emissions aren't the issue here - it's the emissions of specific regulated compounds like NOx. Most emissions from engines are in the form of CO2, which is indeed directly proportional to how much fuel is getting burnt. However, other compounds like NOx and unburnt hydrocarbons (HC) are a product of how combustion is happening, not how much is happening. Basically, the car can choose to trade decreased efficiency (additional CO2 emissions) for reduced NOx and HC emissions.