I presumed this would be the cause.
It's almost always preferable to hold a reserve of the currency you wish to make trades in. Then you can trade freely without consideration to double conversion fees and make bulk conversions when rates are favourable for the direction of exchange.
Of course, not all brokers will support your holding trading accounts in multiple currencies, but there are many that do.
It's certainly something worth considering if you intend to trade frequently in international shares on multiple markets.
🙂