You're standing in a four-lane road and see a car approaching. You're looking at an angle and the lanes are poorly marked, so you can't tell which one it's in. Your observation lets you estimate the chance you need to move at 25%.

When it gets a little closer, you can tell at least which half it's on, the left or the right. Now your estimate is either 0% or 50%.

Closer still and you tell which lane it's in, so now you're sure.