When --risk-free us is used, perf.py converts the portfolio's AUD
cashflows to USD via daily AUD/USD spot (Yahoo AUDUSD=X) before
applying the US Treasury deflator. The reported USD nominal is the XIRR
of those FX-converted cashflows.
The USD nominal can look surprisingly high (or low) compared to the AUD nominal whenever AUD has moved materially against USD over the holding period. This is not a bug — it is the real USD return an offshore