That's assuming the difference is entirely due to bank fees. Exchange rates vary continuously, so a purchase and then a refund at a later date in a different currency will most likely convert to the card's currency at a different rate, especially after 10 months. The bank has no control over exchange rates.

It's very similar to the stock market. If you bought 100 shares in a company in January, would you expect to sell the shares today for the same price as you bought them? Of course not because the price is always changing. Currency exchange rates can have just as much volatility as share prices.