With default settings, light collides with the collision mesh of a model. If the light entity is inside any of the collision boxes, it will not cast light to lightmaps. Default settings also means that every model is lit from their origin point, making this looks rather strange.
Enabling advanced light settings will cause light to collide only with the polygons of the actual model. This also causes light to only collide with the
front of each poly and shine through if it hits the
back. So you can place a light entity inside of a model and the light will shine through.
Static props also allow for tighter control with lighting when vertex lighting is enabled in the compiler. For example, a vertex is lit from the front by default but can be set to recieve lighting from the front and back. A prop will block light in a way that it can throw a shadow on itself, but this can be disabled if the vertices are placed strangely and cause problems.
http://www.nodraw.net/2010/12/lighting-compile-options/