Here is a good way to understand why gravel in the base of a terra-cotta or ceramic container doesn't improve drainage.  Lay a rectangle sponge flat on your hand and saturate it with water.  Wait until the excess water has dripped out (maybe 15 seconds).  Now turn the sponge 90 degrees so it is upright, with the long side of the sponge perpendicular to the floor.  What happened?  Water leaked out, right?  That is because the shape of a container has a lot to do with how much water it holds.  A shallow clay or ceramic container filled with media will hold more water per unit area than a tall, narrow container, which means the longer container will drain better.

When you add gravel to the bottom of a container, you are effectively making that terracotta container shallower, because the water in the upper portion of the container doesn't move easily from a layer of finer-textured materials to a layer of more coarse textured material.  Although this flowerpot or plant container will hold less total water than a similar container filled entirely with media, the top section of the container, where there should be much root growth, will actually hold more water per unit area than if it were filled entirely with media.