Thanks Yankee. Actually, I read this thread before, but I am still confused about the meaning. Some days ago, I read an article in which the writer used this expression. And I failed to grasp what he actually wanted to tell by it.
When a writer makes reference to that, you can probably substitute this: "Thing are not what they seem, and in fact, may be the opposite of what you would expect."
Of course, we'd have to see the writer's actually passage to confirm that.