I once asked about this after running from the F concourse to the B one at ORD to catch a flight which was "boarding," only to find that the plane wasn't even at the gate. I got the standard response that it's difficult to predict future delays and know what to post on the boards. The gate agent seemed helpless to change the incorrect information posted both at the gate and on the departure boards. Therefore, she was forced to make frequent announcements and field non-stop questions as to any updated information.

I don't know how clairvoyant one has to be to post "delayed" instead of "boarding" when a plane is over 40 minutes arriving at a gate. This is one of many questions I no longer ponder after boycotting UAL.