How do you calculate data usages?

My concern is on how the data usages of my data bundels are monitor/measured. <br> Lately I've always run out of date 5-8 days before the end of the month. During February we've been on a boat cruise and i didn't used my phone for more than a week. I've installed an App \My Data Mgr"and according to this app I've only used 600MB in February.<br> How can Vodacom (or any cellphone provider) prove to us ho much data we've used in a month"