City of Tshwane chowing our money and electricity

So my mother who is a pensioner pays her utility bill and buy electricty from City of Tshwane on a monthly basis. Everytime she tops up her electricity mid month they deduct money from her purchase and give her less units. She once went to complain at the office and they tell her it's charges for arrears from her account. What arrears if you pay your bill. I'm just frustrated because someone is clearly *********** pensioners and no one has a proper explanation. Electricity is not cheap and unlike other people who uses free electricity we are the once to pay for it.