It could be a ladle contaminant from batches of other steels. As long as the major ingredients "meet spec", the minor ingredients somehow don't count to the metallurgical engineers, or the sales staff.
I guess I can appreciate Niagra's, um, honesty in labelling. But why publish specifications for a material they purchased from the manufacturer that are different? If you advertise another company's steels because they are a good reputable supplier, why say what you are buying from them, then reselling, appears to be a different material? I begin to wonder what I'm not being told by either party.
OTOH, the following skeptical doctrine in my shop practice will always apply. I can say from experience, and from watching/learning from other smiths, that minor alloying elements can make a big difference at the edge of a blade. Some of us have acquired the habit of treating even correctly paint marked, brand new-direct-from-the-company-untouched-by-human-hands-undiluted-provenance-records-paper-trail-and-all, steels as mystery material until we've made something from it and heat treated it ourselves.