This happens with TM if you're using a Telkom Mobile only data bundle and your device cannot get a TM signal, so it automatically roams on MTN. You do not have an active "All Networks" bundle, so TM shows you the airtime/spend limit reached screen. (Yes I know TM recently did away with TM...
I'm just fishing for anyone else out there who might have a similar experience to this:
My 8ta account tells me my data is about to run out but my Huawei statistics tell me I still have about 600Mb.
I have never before had a significant disparity between my Huawei log and my 8ta account log...