Hello KDC
Tickets are charged in CHF (Swiss francs). You have two conversions: $ to CHF when you buy the tickets and CHF to $ for the refund, with other exchange rates and possibly an additional fee from the credit card provider.
Check the amount in CHF, it should be the same or you could also have a handling fee of 10 CHF from the SBB if it was a manual refund.
Kind regards
Danhiel