Vodacom please explain how 1.9G disappeared from my data over the past 4 days?

Vodacom. What is going on with your data system? I have noticed over the past few months that I seem to be going through more than my normal amount of data, and as I had not really checked on it , I simply let it slip. However the situation is now ludicrous. On Friday 22/05 I received a message from Vodacom stating that I had 1.9G left for the month, which I though it basically should be. About an hour later I had a message stating I had about 750mb left , then an hour or so later I had only about 160mb left. I immediately telephoned Vodacom and asked what was happening? The lady I spoke to was very helpful and told me that there was a system problem and not to worry. Today, I noted I had no data. I immediately telephoned Vodacom and asked them exactly what was going on. I was told that I had gone through 2.8G of data on Spotify over the past week. That is total and absolute tripe. If I download from Spotify I will always do it via wi-fi and in any event if I perhaps inadvertently downloaded 1 or 2 albums over the past week. In any event that would have used perhaps 150mb for both. I do not download videos. My Vodacom data usage apparently for the month was 6.1G. What absolute *******. It would seem that large amounts of my Vodacom data have somehow vanished into thin air. And just in case someone was about to give that standard "answer" that in the case of my wi-fi being slow, the data usage would kick in, my wi-fi averages 90/70