If you group and summarise a report based on any data that can be in mutiple currencies (sales transactions, quotes etc) then the total and currency code applied is nonsensical.
The concept of summarising and totalling values across reports like this is not sensible, as there is no currency conversion happening, the report builder simply totals up the raw database values across every currency, and "guesses" what currency code to apply.
The base currency values are intended for this purpose and can be totalled across a report