Disappearing data

Could you please explain to me how it's possible that I buy a 5GB data bundle on 28 May, which should last 7 days, then on 1 June I should receive an additional 10 GB of data (being the start of the new month), but when I switch my laptop on for the first time on the 1st of June at 08:00, my data balance is 8.63GB.

Let's say, for argument's sake, that I used the full 5GB between 28 and 31 May. I should still have started with 10GB on 1 June - where did the other 1.37GB go?

I did work on Sunday night from 00:00 - 02:00, but that should have been Night Owl data?

I'd very much like an explanation, please. Otherwise it certainly seems as if Vodacom is '*****ing' data from its customers.