Whats going on?!

Im suppose to be paying R320 on my contract every month and if i go over my data and airtime the bill goes up and that i understand. So if i end up with a bill of R500 i pay that and the debit order goes through. But in the new month my balance is suppose to start by R320 because thats what im paying, but it never starts there. When i check my current spend on the day i get my data and everything it says R448 and its always around that total it varries every month. Now how the hell can my current spend have gone from R320 to R448 when i just got my new data and airtime and sms's etc. My bill is suppose to start at R320 each new month and it doesnt. This is ridiculous i feel that im being ******. At first i thought they carried some amount over from last month but i checked my bank statements and they dont they debit the extact same amount as my ending bill for each month. Im so unhappy and i wana know what is this R120 thats added to the R320 at the start of my new bill each month. Ive called customer care several times and none of them could explain to me. Mtn is defintely not what i thought it was, this makes me think it was a mistake taking out a contract with them.