Useless

I have an access bond and paid extra money into it. When I moved the funds out of it, I was charged an exorbitant fee. I queried this with Nedbank and was told that my account went into a negative when the funds was moved. I asked how was it possible as there was sufficient funds but they could not give me an answer just that the system allowed me to do a transfer. A day later I had done a similar movement of funds and was not charged anything at all. So their explanation make zero sense but I have to pay this ridiculous fee for moving my own extra money. Why have this facility then?