You all might be right....
If you are using 3G Watchdog, the way it describes the Android OS (service) usage is data use that it couldn't account for. What that means is if globally you have used 100MB of data, and lets say in actual fact, YouTube used 50MB. But somehow the 3G Watchdog couldn't put that amount of data down into YouTube or only could detect 10MB used by YouTube. Instead of reporting data usage of 40MB less, it will put that 40MB in the OS service usage. Anyway, overall is still what its suppose to be.
However, a note of caution, data calculation apps sometimes gives very different results from each other. Also, apps might calculate the usage slightly different. It does not matter that much except the total isn't toofar off from actual.