In short
Because the position is booked in a different currency from your base currency. The market value is still always shown in your base currency.
Hover over the market value cell (dotted underline) to see the amount in the original currency, the rate (for example "1 USD = 0.92 EUR") and when the rate was last updated.
Bitcoin and other foreign currencies also appear as a small rate line in the holdings area.
Stale rate (amber) means the exchange rates are older than usual. Refresh prices or wait for the next automatic run.