Focal length is based on a theoretical thin lens (a single element lens having minimal thickness). The focal length is defined as the distance behind the lens that collimated rays striking the lens will be brought into focus.
Modern lenses have several elements placed in groups that all converge or diverge light rays. The effective focal length of the entire system of a lens is equivalent to the aforementioned thin lens in terms of the distance from the lens' nodal point to the film/sensor plane, and is measured when the lens is focused at infinity.
On telephoto lenses, the nodal point is beyond the front element of the lens. Wide angle lenses, such as the 20mm lens you mentioned, are mounted further forward than the focal length of the lens and the nodal point is is between the entire lens and the film/sensor plane. A lens of this design is referred to as retrofocal.