I used to explain this with Upton Sinclare's great quote: "It is difficult to get a man to understand something, when his salary depends upon his not understanding it!"

Now, older and perhaps a bit more jaded, I would modify that to "Do not assume that someone doesn't understand something. Assume that he is assuming someone else doesn't understand that something."

Or, more succinctly, "There's a sucker born every minute."