VODACOM - How do my TOP UP contracts vary EVERY month??

Its with great reluctance that I have to give Vodacom a star but it's purely for this exercise. I haven't received a substantial answer as to why my debit order keeps increasing - I have 3 contracts with Vodacom and ever since cancelling the insurance I wasn't aware I was paying, I've noticed my debit order increasing - I have Top Up contracts. The amount increased nearly R100 on the recent account. How is it possible??!!! The service from these guys has gone to the dogs. They originally messed up an upgrade and when I queried the issue, they just transferred the upgrade status to the other contract - when I needed that contract upgraded, I couldn't do it.