Its calculated on mileage and number of days since the last service. It is possible your service indicator was not set correctly at your last service.
If you start to book a service online and the car was previously serviced by Mercedes then the date and mileage of your last service will be...
Home (Robin Lamb)These are in Gloucester and I have used them and had good service.
Also but in Cheltenham
MB&B Ltd – Independent Mercedes Specialists These have good reviews but I have no personal experience
I believe MB recommend a fluid change at 5 years or 77500 miles on the 7 g autobox but best to check with them or look at the ESS service sheet.
Mine previous c class was done as recommended by MB at 5years.