(My billed usage doesn’t correspond to the usage reported by 3GW, why?)
3G Watchdog is based on the “raw” traffic as measured by the phone and so can’t do better than the phone itself. Your operator can apply more or less complex billing rules to that measured usage. 3GW (Pro version only) optionally applies some of these rules (counting in block, ignoring uploads) to help you distinguish between the measured and billable usages, and alerts you based on the billable usage. – you can set up these rules in section “Billing rules” of the Settings screen. Unfortunately there is no limit to operators’ creativity when it comes to billing schemes, and 3GW can’t possibly handle them all.
3G Watchdog leverages Android internal counters to count usage. Depending on Android version, several alternative ways of counting may coexist, and the implementation of some of them may be flawed on your particular device.
Some other factors that can affect accuracy are: the reset time (some operators start counting at an hour different from 00:00, or even use a different time zone); a crash of your phone while you selected a low refresh frequency; an always possible bug in 3GW or in your operator’s accounting software can’t be ruled out, your operator may not always be totally honest (example), etc…
So 3GW will be more or less accurate for you, depending on a number of factors, many being out the app’s control, so don’t expect it to be perfect. In most cases it will be accurate enough to really help you to anticipate and master your mobile data usage. Your part of the job is to try to know the billing strategy of your operator (which 3GW can’t guess) and tune the billing rules accordingly, so the usage reported by 3GW will match your operator’s one as closely as possible.