Invoice for 9 days of new data contract is more than my monthly subscription.

I took a new data contract out on 7/09/2017 for 10 Gig day and 10 Gig night for R339 PER MONTH. I received my first invoice 21/09/2017 with amount of R424.52 . I used the modum for first time 9/09/2017. ON 14/09/2017 the Pro-rata data was finished and had to wait for 17/ 09/2017 for the 10 Gig monthly data. In phoned 808 and i've been told the difference is for the Pro-rata data R132.17 and i must pay the full amount.

My question is: Why must i pay a full subscription (For 10 Gig data) for 9 days and for Pro-rata data if i havent received anything that is in my subsription?

No one at MTN Store Jacaranda Centre Pretoria or Mtn Call centre can give me an answer.