I don't know if your sample size is large enough to draw a valid conclusion. When comparing USA-issued American Express versus VISA cards the exchange rates have been very close. However, for a card billed in Euros I believe American Express converts other currencies first to USD then to Euros. This does not mean there is a markup at both steps, only that there is a double conversion. In a period when the dollar is weakening this might cause a poorer rate than a direct Franc to Euro conversion.