The orbits of Pluto and Neptune do overlap. However, because the orbit of Pluto is tilted at an angle to that of Neptune, it can appear from your perspective that Pluto is always outside Neptune. A neat way to see this is as follows: Choose "Favourites | Solar System | Outer Solar System" and rotate your view so that you are directly over the north pole of the sun looking back down on the solar system. The orbit of Pluto will appear to be completely outside the orbit of Neptune. Now blast off using the elevation button until you are about 3000 AU above the sun. Use the zoom button to enlarge your view so you can clearly see the two orbits. Now you can see that Pluto's orbit does come within that of Neptune. The "correct" view of the orbits can only be seen when you are very far away, and the effect of Pluto's tilt is small compared to this distance.