Officially the GB, MB, and kB unit symbols are based on decimal arithmetics (1 GB is 1000 MB, 1 MB is 1000 kB and so on) whereas the GiB, MiB, and KiB are based on binary calculus (1 GiB is 1024 MiB, 1 MiB is 1024 KiB, etc…).
In practive however this standard is largely ignored. All ISPs use GB / MB symbols but for some 1 GB is 1024 MB and for others it is 1000 MB.
This is very confusing but unfortunately there is no easy solution. 3G Watchdog uses binary arithmetics (1 GB = 1024 MB, etc…). If you find out that your ISP is using decimal arithmetic (1 GB = 1000 MB, etc…) you must adjust the value entered in 3GW for your quota as follows:
value entered = (ISP value in GB or MB) x 0.9765