Strange Data Usage

I have spoken to a Telkom consultant about data that expires 01 Jan 2018, against data that expires 01 Feb 2018, my problem is that the data that expires 01 Jan 2018 (doesn't get used), but my data that expires only 01 Feb 2018 get used. The first consultant guided me to put roaming on, on my router and said that will cure the problem, roaming was put on and that didn't cured the problem. The second consultant advised that it is how it is, first in, first out, then i said but how is it possible that you can use data from the portion that expires first, then the other which expires a month earlier, doesn't make sense to me. I am loosing 500MB of data because of this. Please see below: Recurring Telkom Mobile DATA GPRS: 9624 MB remaining Expires on Thu Feb 01 2018 <<-- Used first, but only expires 01 Feb 2018

Included All Networks Data GPRS: 500 MB remaining Expires on Mon Jan 01 2018 <<-- Used after the above is depleted, but when you go over to the next month with the above, then you loose the 500MB by default, doesn't make sense to me.