This is because in the United States the sale tax does not get shown to the customer until they checkout because different states have different sales tax rules.
In the UK and Europe the VAT needs to be shown with the price prior to checkout. So what’s happening is because the price is shown in dollars it’s not adding the taxes on so the dollars are being converted to the Pound Sterling and then the 20% VAT is being added onto the shown price during checkout.
Americans are not paying the shown price, soon as they get to checkout their sales tax is then added on too because each State in the USA has their own sales tax rules.
Using your example, $79.99 is the price prior to sales tax. They convert it to £61.35 (the exchange rate at the time) and then the VAT is added on, 20% added onto £61.35 is £73.63
What FAB needs to do and do pronto is convert the currencies and add the VAT on prior to the checkout. Showing the price in dollars without the sales tax / VAT is causing confusion to none Americans.