Top up credit

I bought £5 top up credit. With no any reason it became £4.88; I stayed within the limits of my plan and did not use roaming. Now I can't use all of my credit, as purchases are either £3 or £5.

MarjanC1 just to clarify, did the credit from your topup get used or was the price you paid £4.88 instead of £5?

I paid £5, and after some time it became £4.88