MTN are "*****ing" my data!

MTN continue to "*****" my data every month - some of it just simply disappears! I have been watching this for a few months now - I take a screenshot of the airtime, sms and data balances the day before I am to be credited with the monthly amounts as per my contract and then check them after I receive the SMS advising me "....you have been credited with 500MB Data, 200 Minutes and 200 SMS." Yesterday (26 November 2018) my balances were: Anytime Minutes: 82.03 SMS: 398 Data: 631.91MB . Today (27 November 2018), after I was credited, my balances read: Anytime Minutes: 275.21 (I did make a few calls so this looks to be correct) SMS: 400 Data: 1000.00MB. So my question is, what happened to the missing 131.91MB that have simply disappeared? My calculation says that 631.91 plus 500.00 gives a balance of 1131.91MB. I called MTN who tell me that I have applications running in the background that are using up the data. What utter *******! First of all, I only use wi-fi (NOT mobile data) at home and I have been at home and only on wi-fi since 5pm on Sunday (25 November 2018). Secondly, if I had a balance of 1000.00MB when I was credited on 27 October 2018, a balance yesterday (26 November 2018) of 631.91 this means I have only used 368.09MB in a month, so how could I possibly have used 131.91MB overnight??????? This is ludicrous to say the least and I am sure I am not the only MTN subscriber experiencing this "theft" of data. MTN need to come clean on this issue of helping themselves and stop just "*****ing" the data I have paid for !!!