Ned,
Probably the same way I "hear" my wife nagging at me but never "listen" to her! It happens in maintenance often as well unfortunately. Several people can look at a loose nut or bolt expecting to see it tight and say that it is when it is not. I always preach "rag maintenance". If you are wiping something off with a rag it doesn't just make it clean, it shows you by feel that something is not right instead of just "looking" for it.
Max