Piping line stock does not use material density

In Creo 4, M080, one can create a pipe line stock and choose a material source file. Yet the option of entering a weight/length option still exists. This gives an erroneous pipe density and mass. The material file includes the material density, so why does the piping line stock not use the  correct values and relies on a weight/length value, that has to be recalculated if the pipe diameters are slightly changed.