In Salesforce CRM Analytics, when dealing with multi-currency environments, the system relies on the organization's corporate currency setting for reporting, unless explicitly overridden. In this case, even though the Opportunity data is stored in GBP in Salesforce, the dashboards are showing USD because the corporate currency for the org is set to USD. This behavior is expected unless currency conversion logic is implemented in the datasets or recipes.
The corporate currency serves as the default for currency fields in datasets and dashboards unless configured otherwise. This can be confirmed under the "Manage Currencies" settings in Salesforce.