Steve explained the cross section pretty well. Many bowyers make their limbs with a trapezoidal cross section. Most of them make the back narrower.
As Dave stated, I trap the limbs on Shrews for two reasons. the first is to bring the bow to correct draw weight and fine tiller the limbs.
The 2nd reason is to remove limb mass, especially in the outer portions of the limb. This helps some with arrow speed and softness of the bow as the arrow is released.