The brakes are part of CBS. CBS is Condition Based Service. So depending on the conditions )i.e. how you drive) the system will base the estimations when it views it is time for service. As previously mentioned, it really doesn't know with 100% certainty when brakes need to be serviced until the sensor tells it. Up until that point, CBS is using various factors to determine when it thinks it will be due.

Maybe you are much lighter on the brakes than the previous owner and thus the mileage is not being reduced as you are putting far less wear on them.