Why can't MTN pass a simple credit of R13.15?

On receiving my MTN monthly statement invoice dated 21/3/2015 I noticed that I was not given the Recurring SMS Bundle 100 discount of R13.15. On 26 March 2015 I phoned their accounts dept and was given ***. On receiving my April Invoice dated 21/4/2015 I again noticed that the R13.15 discount had still not been credited to me. On 24 April 2015 I sent an e-mail to Customer Care enquiring when am I going to be given the promised credit of R13.15. On the same date I received a response from Luciano du Plessis explaining that \more than one customer was effected due to this error on the invoices a mass credit will take place in the month of May 2015 so on the May invoice this amount will be credited back to you as promised". I was given another reference number of ***. <br> Well"