How my Ertiga reached mileage of 17.8

One week ago I bought Ertiga VXI Limited Edition 2018, I go with Limited Edition cos of all black interior n alloy wheels. The next day we 8 members including 3 kids went to a short trip and in the mid its petrol light starts blinking so we filled more fuel in it and techometer showing average of just 8.5 which looks terrible to me after 6 days I visit dealer showroom and met with their service manager and reports the low mileage problem he sends their technical person with me and we both goes to a drive he starts driving and to my amaze the mileage starts improving from 8.5 to 12 13 15 and finally 17.8 which is more than the specified mileage in brochure then he gives me some tips like driving in the same gear etc and now I am very much satisfied with my ertiga