Mweb
MTN - 3G - carry over of data to next month

For the previous month i had a base cap of 1G plus carry over of approx 500 meg totakl 1.5G.
I used approx 775 meg in the new month. Today my account shows for the new month... 1G... base plus ONLY approx 225 meg carry over.
So... Does Mweb fork on a LIFO or FIFO regarding the usage??? I have a direct contract with MTN, on this contract it is definitely a FIFO usage.
I would like to know why MTN is "taking" my un-used data!!! Surely this is NOT a correct procedure