From: GoldMoney - Frequently Asked Questions
http://goldmoney.com

Why is the buy rate dependent on the currency that I use for payment?
http://goldmoney.com/faq-why-buy-rate-dependent-on-currency-used.html

The global bullion market is priced in US dollars, which means GoldMoney must first convert any other currencies into US dollars before it can buy gold and silver in the market on behalf of its customers. Therefore, GoldMoney charges a slightly higher fee for non-US dollar purchases in order to cover the costs of converting those other currencies into US dollars.