The €1 could have gone on services that are charged to your cash credit balance - e.g. sending MMS messages (30c each), international SMS (25c each), international calls etc.
You seem to be using an iPhone and it can silently send some UK SMS messages to activate the phone, FaceTime, iMessage etc. and these cost 25c each - this is due to the stupid way that Apple do things.
If you want to confirm where your "missing" €1 then contact 48 support and they should be able to tell you.
In the meantime if you need to buy €20 Don membership then you'll need to topup by and extra €1 before you can do this.
Hope this helps.