More Data Deducted than Used

The mobile data charges on Vodacom has become absolutely ridiculous! And it is becoming more and more expensive by the week. I have caught Vodacom out on several occasions charging me more than I am using. I have seen how Vodacom charges me 38MB for a text only email, and 12MB to just open a text only email. I have checked my data usages at different times, and it does not at all correspond to the deductions on my Data Bundle. Vodacom has used the excuse that it is background apps that are causing the deductions. However, on my WiFi this is not a problem. My data usages are charged normally and emails are charged in KBs, and not MBs as on Mobile Data. This problem is also something that every person I encounter experiences, and complains about. Therefore, it cannot be only my phone that is faulty.