What is the use of buying a data bundle?

Good Day <br> <br> At 19:21 on 21/03/2016 I had the following airtime on my phone: R27,70 all inclusive and R90,00 non-expiry. I also bought a 100 MB rush hour data bundle at that same time (costing me R10,00 and so I had R90,00 remaining). <br> <br> I browsed the internet (not using my phone for any calls) from 19:21 to 20:15 when I checked my airtime status once more. It showed that I still had 46,27 MB remaining on my data bundle, but I was left with only R75,42 Non-expiry airtime. This does not make sense to me: How can MTN take airtime for internet usage while I still have an active data bundle? Can someone explain this to me, because it seems like MTN is *****ing from their customers. <br>