The engine also has to reach it's own level.........what I mean is that if the oil level was checked immediately after the service, and then run for a few miles, it may need a small top up. Too often garages judge how much oil to put in by measuring on the dipstick, as opposed to actaually measuring how much oil is being put in in relation to how much the engine will hold. When I do my service on the GTA, which takes 5.9 litres, there is about 200ml left in the container, but the dipstick reads full. After starting the car and letting it run for a minute or so, the level will have dropped slightly.
EDIT You posted before me, so please ignore all of the above, as you have already gone through those checks.