as of today while you import step/x_t file to creo you have two options:
1. Use Templates. in this case should the template contain density Creo ignore density coming from step/x_t file and therefor mass calculation is wrong.
2. Don't use templates. in this case Creo populate the density coming from step/x_t to file and mass calculation is correct.
having the ability to decide whether to keep density coming from imported file, using import profile/config/interactive user selection, will reduce errors and simplify data exchange process.