Immigration Health Surcharge

Hi! I'm trying to finish up my visa process and I got to the Immigration Health Surcharge part. The website says it costs 624 pounds - but then when I go to pay it, it charges me 938.72 USD, far more than the exchange rate would yield (624 pounds * 1.27 = ~793 USD). What's the deal with that? Any help or explanation would be appreciated, as the government provided contact link is an e-mail that no longer exists.