I had this very discussion with Thomas Steenburg (in Search of Giants) over coffee this afternoon. In his extensive research in Alberta, he has come across several sightings of very tall creatures in the Bighorn Dam region, spanning several decades, then ending abruptly in the mid '80s. His first thought on this is that some of the sightings were from a point of view lower than the animal, and that angle could make the figure appear taller. The fact that the reports of extreme height ended in the '80s, with no more anomalous reports since, could also indicate that one or more very tall individuals really did inhabit the area, then died out, or went to even more remote areas, avoiding further encounters.
BTW, Thomas' 30+ years of research, mostly in print and audio formats, is in the process of being converted to digital files, and will soon be appearing online at his new website, Thomassteenburg.com , starting in the early part of the new year.