The reasoning here is so that the parts output agrees with the path length. The path length is always calculated between the start point or “beginning” of each part. That doesn’t really make intuitive sense for structures, but it comes into play with pipes. Take this example:
What would you say is the shortest network distance from MH1 to MH2? I think we’d all agree that it is 50 feet. What about the shortest network distance from P1 to P2? I would again say that it is 50 feet, but that depends on where you begin and end the measurement. And for one more example, what about the distance from MH1 to P1? My guess is that most users would probably expect the path length to be 0 in this case and not include the length of P1. By not including the last part, the ambiguity is removed and the validity of the name “shortest network path” is held true in all instances. So I decided to leave it up to the user to append the last part if they want to.