Now that is unlikely too, although manufacturers do screw up once in a while. However, to be sure, could you verify your exposure times next time, and make absolutely sure you are exposing for 400 ISO. In addition, you may wish to have your HP5 run along doing exactly the same exposures at the same F stop / time settings. In addition, shoot one of your supposed Rollei Retro 400 films at 100 ISO setting. If it turns out good after development, I guess you have definite proof you have a wrongly packaged ISO 100 film.