Account is always in arrears.... Why????

I've made an EFT to vodacom for account number NM016032-0 ON THE 3 APRIL 2019 FOR THE AMOUNT OF R1149. 14 as per my previous statement, this month again, vodacom sends me a statement reflecting R1220. 01 in arrears, why, where does this amount come from??? I'm really tired of paying for arrears when I've already paid everything. By my calculations, I'm only due to pay the monthly fees of R579 for ********** 757 and R579. 00 for ********** 128. This is ridiculous that vodacom can just charge me for balances brought forward when the account has been paid.