for this particular reason i'm running my own application based on IB's API, where within a click i can see PnL based on worst case liquidation(commissions included), best case and based ol last trade.IB's TWS PnL is too good to be true most of the time.
try to liquidate portfolio of 20-50 small stocks with decent spread-you will see the differenceUnrealized is always accurate as well an individual underlying unrealized and realized PnL.