Combo Bid/Ask Calculation in TWS-IB

How does IB calculate BID and ASK of the combo in watchlist? It doesnt seem to add up any way.

Isnt it suppose to be ASK1- ASK2 and BID1-BID2 OR ASK1-BID2 and ASK2-BID1 ?
 
The combo itself has its own market. So if someone has a buy or sell order on the combo then the combo bid/ask takes precedence over the derived bid/ask you'd get from the bid/ask of the legs.
 
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