MTN charging for data but you have bundle data

ONCE again MTN is *****ing from me and now they trying to find some excuses on what went wrong. I have a Tablet with 1GB month data. This tablet is off most of the time, and when in use on the WifI on my uncapped line. So my December billing comes and indicates that i have used over R500 out of bundle data. With is impossible, so I log into my MTN active , and here it indicates that I have 1.4GB (400MB Cary-over) data in January, I also then do a balance check this indicates the same, and also the monitor installed to check data usage indicates the same. SO I am like why do I have data left over, but was charged for data usage. I phoned MTN, and the Consultant could not explain this to me either he was shocked as me as he confirmed my data carried over but was charged for data usage,SO I asked him to check another invoice fro last year. Same thing here he also found was charged for data but still had bundle data left. So I they log incident, 3 weeks later I phone back to find out that was just closed, stating I used this amount and was billed correctly, but still they can explain why data got carried over. I logged complaint, now they just email me list of URL. Still no explenatios