Thanks for the input to everyone, since my last post, yesterday. Helped steer me in the right direction, somewhat. I've carried out a detailed reconciliation, and think it's simply
an error in naming the narration
Firstly, to my surprise, when I logged onto my Vox profile a short while ago, I found that the "Data available" amount had gone up[!], not down, having downloaded some podcasts last night.
By the way, I don't have a "Fatpipe" a/c linked to mine, which is only the Gig4Gig & Bolt-on combo.
The "Data available" amount now shows 15115MB.
I manually added up my Daily Usage breakdown for August 2014, and get 15,06GB off-peak so far this month + the 41MB I mentioned yesterday = 15.11GB .. as close as is needed to confirm that the narration added in relates to the TOTAL of DATA USED (for the current month), not Data "available" which caused the confusion.
@Voxy, maybe @cvanwie (Chris Van Wie, Solution Architect, Vox Telecom Limited) can get someone to change that narration so it's clearer that it relates to what's been used, and not available.
My usage makes sense, as I tend to go through about 27 - 30GB a month, on the Bolt-on.
I don't feel it's necessary to PM my account details, as everything seems fine, but if that change could be made, if Chris agrees, then I'll leave it at that, and will check for a change in my profile, when I log on in future.
Cheers.