WooCommerce reports, and the reports provided by 3rd party plugins, are not "currency aware". When you run a sales report, the totals you see are a sum of all the order amounts, regardless of the order currency. For example, if your shop's base currency us USD and you have sales for 100 USD, 100 EUR and 100 GBP, the basic reports and the reporting plugins will show you "300 USD" as the total.

The Currency Switcher currently addresses this by converting each order total to the base currency (USD) and by overriding the basic WooCommerce reports, so that the totals displayed are correct (i.e. 100 USD + 100 EUR converted to USD + 100 GBP converted to USD). This operation can take some time, which is the reason why you might not have seen the values straight away.

Please note that reports added by other plugins are not affected by the above fix and they will most likely still show incorrect figures, "mixing up" currencies. To fix this issue, the reporting plugins have to be updated to take into account the order currency, and either allow to filter the reports by currency, or calculate their totals after converting each order value to a specific target currency. This is something that the original plugin authors should do and, of course, we can provide them with assistance and examples on how to achieve that.