Well a lot of people look at things but don't actually see. I imagine the psychology behind your example is that you were so convinced that the field was open (having spoken to the guy on the phone) that you weren't looking for confirmation of that. You - therefore - subconciously ignored that there might be other useful information on the signal square and only took in the information you expected/wanted to see. Probably made more difficult by the fact that you had other things on your mind other than actually looking for the information - such as controlling the aircraft at reasonably low level etc.
Glad it all turned out well in the end.