Official comment
If you're using Multi-Currency, your global reporting currency (set in Global Settings) is referred to as the "Base" Currency.
Reports do not convert currencies, but can display different currencies in report tabs and show base currency values alongside the project currency type data.
You can ensure custom currency fields expose their base currency value.
Take a look here:
https://success.clarizen.com/hc/en-us/articles/360012180233-Multi-Currency-Support-in-Reports