Airtime and data bundles

I am on Vodacom prepaid cellphone, but I have notice that there is a problem when it comes to billing. I buy my self R12 airtime and data bundles to access my whatsapp and at time internet. But 1 thing I have noticed once my 3G for data is on also my airtime gets billed simultaneously as my data bundles. Can someone explain how can this happed as a result I have been comparing this with my wife who is also on Vodacom she experience this.