Most airlines use the IATA day rate to convert. I can't particularly recall if this is something I've tried before, but I only noticed because I was playing around with dates on the app and website around the same time. I thought A3 also used IATA conversion rates (whenever I've paid an upgraded voucher cost for UK APD, it's been in Euros at the right conversion), so maybe this is a recent change. We're talking about 5.5% difference, much more than any credit card would impose.