I think the difference of refund you received was due to using Amex. Anther member posted a thread about this couple years ago. She was upset about getting less and contacted H, who told her they process Amex differently. Something along the line of having Amex purchases/refunds processed in UK instead, hence having to go through currency exchange twice.
They really should warn you about the difference of cards when paying but u know how things are in Europe. Or perhaps the cashiers aren't aware of the fact.
Yes if you look at the global blue site it says that they process Amex refunds differently. This isn't my first time going through this. But in the past if you clicked on the details of the transaction on the Amex site they would tell you the amount in pounds so you could figure out the conversion rate. Now they don't tell you. I can back track from my receipts to the amount in euros. I suspect they are giving an even worse change rate than in the past and that's why they don't want us to be able to figure it out easily. Time t shop around for a new card.