Gas mileage decreased post 4th service

Hi All - Need your views. My car went in for the 4th yr service. Its got 35K km on the odo. As part of service, beyond oil change, they also replaced the air filter and spark plugs.

For a few days, the gas mileage in every trip increased by about 20%. However, its been about 15 days now since service and the mileage has been going southwards. In every trip, I feel the mileage is lesser than norm by about 15 to 20%.

Thoughts whats going on?

Cheers