No, no, Mercedes does not plan for the car to be serviced nearly that often.
The FSS is reset to 10K miles when the car is serviced. (You can also do this yourself - no tools are required.) It then counts down toward zero. At zero, the car gets serviced.
It doesn't actually count down mile for mile. It takes into account the number of starts, how hard the car is driven, number of hours running, and more. In typical city commuting use I'd expect about 12K miles to elapse before you get to zero.
That's why I suspect you need a "B" service. I'd guess the car had an "A" service in the vicinity of 12K miles, and 12K miles later requires a "B."
If you insist on being obsessive, I'd recommend using a synthetic oil (which is what MB requires for FSS cars) and changing it when the FSS counts down to 5K miles. Then take it in for the A or B service when the FSS hits zero. Changing the oil 100% more often than the manufacturer requires ought to be sufficient overkill...
Definately get the service booklet. In the back it lists the services required for A & B - gives you an idea as to what is done at each.