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.