It's likely the Windows system is burning the .dmg file to the disc and not burning the image it represents. Thus you're not burning a Tiger image but merely a file. .dmg is a format used by Macintosh systems and it's unlikely the burning software you're using understands it as an image format. Maybe someone else can advise if there's a Windows, I assume Windows when you say PC, based burning software that understands .dmg files.