Mobile data and airtime disappearing then reappearing

<p>Scenario – Purchase R30.00 MTN prepaid airtime, did the whole *141*2# and got a 100MB for R15.00, barely use my phone and about an hour or so later I decided to check my balance and to my surprise things aren’t looking right e.g. now my airtime went from R30.00 to R15.00 and now I have R9.00 and not to mention my data went from 100MB to 90MB and now I have 50MB… So frustrated I thought well lets restart the phone and once again to my surprise my balances are back to normal, so what I would like to know is WHY is this happening???? I have been with MTN for over 10 years and I have always been happy with the service I received but now I feel you guys are taking fat chances, can someone assist me with a good enough reason as to why this is happening