'Top-ups' to a pay monthly account
When you have a 'pay monthly' account and you top up, you are topping up a number of minutes and NOT a value.
Your account is set to up a highly discounted rate and so, when you top up £10 for 500 mins or £20 for 1000 mins for example, we add the number of minutes to your account, and this may well show as a different value due to your low tariff rates.
Please rest assured that this ensures you get the number of minutes you have purchased, whatever happens.
I hope this helps clarify.