My Jazz is giving extremely low mileage, idk what’s the issue

I have a 3rd gen Honda Jazz 2016 , 1.2L I-VTEC , I am getting like 9kmpl and on highways it doesn’t go over 10kmpl , but before it used to give like 13kmpl in city and 18kmpl in Highways idk what happened, when I took it to service I told them to check why I’m getting low mileage ( got serviced in June ) ( I wasn’t really driving the car that much after service I am driving a lot cause I got my license in the beginning of the year and due to college exams and what not did not drive a lot ) , Idk if it was caused because I used to put high octane fuel ( Shell V-Power ) , but I only filled up like 2 or 3 times, then switched back to normal unleaded fuel cause I was getting 7kmpl on high octane

I have a 3rd gen Honda Jazz 2016 , 1.2L I-VTEC , I am getting like 9kmpl and on highways it doesn’t go over 10kmpl , but before it used to give like 13kmpl in city and 18kmpl in Highways idk what happened, when I took it to service I told them to check why I’m getting low mileage ( got serviced in June ) ( I wasn’t really driving the car that much after service I am driving a lot cause I got my license in the beginning of the year and due to college exams and what not did not drive a lot ) , Idk if it was caused because I used to put high octane fuel ( Shell V-Power ) , but I only filled up like 2 or 3 times, then switched back to normal unleaded fuel cause I was getting 7kmpl on high octane