In Overprint Preview the objects look thicker. Why?

In Overprint Preview the objects look thicker than in regular CPU Preview.

Take a close look at any vector object in Illustrator, change the view to Overprint Preview and you'll see the object becomes thicker as if you've added a stroke. 

Am I the only one seeing this? 

Is this supposed to hapen?

If so, I wonder which of the two is more accurate in terms of the printing result.

In this example, the one on the left is the Overprint Preview, which as you can see is thicker than the one on the right.