The currency used across all reports within one group is defined by the Group settings. If the data sources used in these reports contain different currencies, they are automatically converted to the group currency. 

To do that, Nugit is relying on exchange rates from an API connection to Openexchangerate.org.  The rates are updated on a weekly basis.
As Nugit doesn't store historical rates, data from previous reports may change, depending on the most recent exchange rate.