Newest Data used first ???

I have 10GB per month data on my phone, and the data expires at the end of the next month.

So there was 10GB at the beginning of Jan which expires at the end of Feb.

I was running short at the end of Dec and bough an additional 3GB which expires at the end of Jan.

So at the moment Vodacom is using the 10GB bundle, which expires at the end of Feb, first, while the 3GB bundle, which expires at the end of Jan, remains unused.

The 3GB bundle will probably expire untouched at the end of Jan.

What nonsense is this, or is this just what one should expect from Vodacom ?