2001 Other Mercedes Benz Models RELATIONSHIP BETN. MASS AIR

Just boughT a 2001MB E-430 4 MATIC from a dealer with 65k miles. Car is mint, drove it for about 700 miles and monitired MPG to be an average of 22mpg in mixed driving condition. Check engine light came up and dealer sent it to a specialized reputable MB shop and determined that it is the mass air sensor that needs to be replaced. Repair was done and same day filled up gas tank and drove about 160 miles and car did 25 MPG ( after initial repair, was impressed ). Five days after filled again and seemed that MPG had dropped to about 20 MPG, TAKING INTO ACCOUNT USING SAME GAS AND SAME DRIVING STYLE. The question that I have, is there is a chance that the air mass sensor was not installed correctly or needs adjustments to improve the mileage. Or could it be after market part that was installed and not performing as the specs so it lowered the MPG. Also need to mentione that just filled up tank this one time which experienced this decline, more monitering will get performed to confirm in the future. Again all what I am saying is before replacing the sensor MPG was high, after replacing the sensor MPG had declined lower. Is there a relation betn the repair and the MPG that seemed declined. Pls advice and thank you so much. EH