The author of the book I'm currently editing consistently does a bizarre thing with em dashes.
She basically understands that one of the em's proper uses is to indicate an interruption or a sudden change of thought.
However, her execution of the em's function is literally halfhearted. For example, she writes, "These differences—as in concepts of time--need to be understood to make social interactions easier."
Why the double dash rather than the em to finish the interruption? All I can think of is that it's her way of first saying, "Stop and take note of this break," and then giving us the go-ahead to proceed to the rest of the sentence.
I have a feeling that she's never run a red light.