I can only guess but ...
It sounds as if you have a price-plan that either has an automatic bundle of extras on or where you pay extra for it. So there is the standard payment you pay regularly for your calls, and a further payment paid for this extra, which presumably allows you cheaper calls or data or whatever whilst it lasts.
But it is possible the extra is only usable in the UK. When you roam it remains untouched and the cost is taken from your standard payment instead.
Have a read of your contract , it probably tells you there.