Once-off data given as prorata

Good day

We took out a new Contract with Vodacom and as advertised it comes with 10gb once-off but I only received 5gb and the shop attendant said they cannot help with this as its from vodacom head office, so we called 135 and did the security check just to be told it’s prorata, but prorata on a once off is like going to Car dealer and buying a car and only getting half the thank you gift as it’s the 15th of the month, the rest of the contract was fully allocated just the once off was prorata?

Can anyone explain why this is? As the customer care told us that is how they do it. That is all she can say