3 phones one account but 3 different prices for Data Bundles

I would like Vodacom to explain their data bundle charges. I have 3 phones with Vodacom under one account and each phone have different prices for the data bundle. In January I bought a 3GB data bundle for R199.13 for 30 days. for phone ...3891. This month the price for the same phone costs R229 for 3GB for 30days , R29.87 more. Now for the other phone ending ...2447 I have a recurring 5GB data bundle monthly for R152.17 at 50% discount. If I were to buy a 3GB data bundle for this phone I would be paying R189 for 30 days. So why the discrepancy amongst my phones. We are not talking about Crude Oil here where petrol price can vary every month. Does Vodacom have a "Charge Center" where they have consultants to monitor customers data usage behavior or computer giving monthly data usage analysis (Business Intelligence or Data Mining) and charge accordingly. Vodacom please explain, ICASA has ask Service Providers to lower prices for data.