I didnt think this still happened on top ups .
It most certainly does. Every day members who do want to buy a goodybag report this exact same problem. It's caused by their phone having outstanding background tasks to do, but no credit to do it with. So the moment they have credit, their phone will immediately go ahead and perform those outstanding tasks, which is likely to use a penny or two of that credit before they are able to buy the goodybag.
But why is it that members choose to add credit and then buy the goodybag from the credit instead of just buying the goodybag without adding credit first?
Is it because nobody told them there was a better way to do it? If so, everybody should be supporting https://giffgaff.com/top-up :
The link in the hint takes them to https://giffgaff.com/buy/goodybag where they can buy the goodybag direct from their payment source, so there's no danger of the phone using a penny or two of credit before being able to buy the goodybag.
Whilst your post may be factual for some cases, to extrapolate and conclude that it is true in all cases, is nothing more than arse gravy.
My recent "one pence disappeared" happened while my sim wasn't in a phone.
How does that fit with your explanation?
Fewer graphics more facts.
(and analysis therof).
I think @thunderdragon
Was by far the closest to actually explaining what happens.
At least he hasn't explained away a real phenomenon with "oh it's just you being a thicky and not understanding apps, and how they update in the background.
(whilst ignoring that people report the penny glitch happening with data turned off, or even (as in my case) sim not even in phone.
It's not exactly the end of the world, but please.
At least acknowledge that it happens!