In my experience some airport ATMs (IIRC the ones associated with Amex and Thomas Cook, including at LHR, ZRH and GVA) give customers two options when withdrawing money:

Option 1 is to withdraw at your own bank's exchange rate plus the fee for using an ATM outside the Euro area in my case. Option 2 is to withdraw at a rate suggested by the ATM owning bank (plus a fee?).

Maybe in this case choosing option 2 included some hideously disadvantageous exchange rate?