Upgrade Means Being Billed Twice

I have always upgraded with Cell C and I have never been billed twice.<br> This time I had to be billed twice simply because at the beginning of the month I received 2GB of data which I didn't even use and then on the 9th when the upgrade took place I got pro-rata data and got charged for it. Why when upgrading am I getting extra data, which I did not request? I am just upgrading on the contract I have, shouldn't that just continue from what I have till the end of the month, because I am not taking a new contract?