Ahhhh OK... I understand now... you have the "Show History Rates" box ticked on the "Tools -> Preferences -> Fiat" window don't you?
This does exactly what it says... it shows the "Historical" rate for each individual transaction... ie.
what that amount of bitcoin was worth when it was sent! The fiat value for the overall balance, shows you what the total value of your wallet balance is worth
NOW.
If you had a 1 BTC transaction from Dec 2011, you'd see it was worth like $2... even though the total balance today would show $5800+
