IB Market Value vs FX Portfolio

Hi, Can anyone advise please, help me understand the difference between the "Market Value" and the "FX Portfilio" on the Accounts Display. They both show a listing on currencys. I understand what they refer to, the FX, is a "virtual position", as if it were a "contract". What I can't understand at the moment is:

1. how the 2 can differ so much. Why isn't all the "Market value" referenced in the "FX portfilio"? If the FX portfilio doesn't cover all the holdings, what is the point of it, I don't "get it". I suppose the FX encompasses a history of past trades somehow?
2. Which reflects the accounts sensitivity to currency fluctuation?

Any help is greatly appreciated.
 
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