The Galaxy Tab 10.1, 10.1N, 10.1v, and possibly the 8.9 don’t work well with data usage counting applications. 3GW leverages functions provided by the Android programming interface, and apparently they are incorrectly implemented on these devices (see Android issue 19938). There is nothing we can do about it (we have no access to the faulty code). Let’s hope that Samsung will fix this in future upgrades of Android. Sorry for the inconvenience.
The Pro version since 1.10.0 has a workaround: you can check the “Calculate global usage from usage by app” option in 3GW Settings > advanced to tell 3GW to use the sum of all application usages as the source for global usage. It’s not a perfect solution though, as usage by app is generally less accurate than global usage.
Update Sep 15, 2011: if you are ready to take the risk of flashing a new kernel (and assume the consequences), custom kernels available on this page, from version 126.96.36.199 (Aug 26, 2011) or more recent, allegedly fix the problem for the Galaxy Tab 10.1 (not verified though). Instructions to flash the kernel are included in the page. You also need to uncheck the 3GW Settings > Advanced > “Use pre-Froyo traffic counting interface” option for the counting to be correct.