When you say "about 60mb" - Is that how much you estimate you used (Eg: Downloaded a 65MB File), or how much your network status shows?
If the first option, you have become a happy victim of what I call "leaking"
Picture the following situation:
You connect, log on to Facebook, spend 2 hours on chat (Hey - It happens

), check 2 e-mails on GMail, and are about to log off when you decide to check your data usage.
Technically, Facebook chat and 2 small e-mails would use... what.... 10mb? 20mb max?
You check your data usage, and.... "Used: 280MB"
Gasp! Horror! Vodacom has robbed you!!!
.... Havn't they?
What you DON'T check is that your Windows Media Playlist of 800 songs is now magically fulled with album art, your anti-virus is strangely up-to-date, your uTorrent oddly shows that your seeding ratio is significantly higher, and, when you shut down, Windows decides to randomly install 25 new updates (Where'd those come from?)
This situation is primarily due to the fact that everyone in the major first-world countries (IE: USA) live in an uncapped world, and assume the rest of the world does too (Wait.... You bought MB... ? What? I turn on my computer, and just play online games... I don't know what you mean you "connect" .... ?) - For them, random downloads / updates / etc don't affect them at all. Unfortunately for us - It does (As it has affected you)
Welcome to South Africa, and the internet life of a third-world country.