please check whether they change the the same rating oil coz factory fitting oil is mineral 10w 30.

2ndly, check whether u r checking the average fc@instance one, n remember to reset it coz the reading will be average of since day one unless u reset it.

3rdly, just probability, even with same driving habit n route, but its not the same idling time coz idling time also can cause up to 30% diff in fc(including idling at traffic which not the same everyday), so,there willl be a plus minus for every fc.1 to 2 litre plus minus, is normal.

4thly, another way to get better mileage that even me dont do it nowadays is to wind up windows, driving less than 90kmh :-). Have u guys try this out in cfe? Its efficient, but not comfortable. Thats why i never suggest this method. But for fun sake to check how much the compressor take out the power n fc, give it a try for a full tank.