None of these numbers would be right in your example. Not even zone 8, because the plant is hardy there for only 90 percent of growers. The minimum hardiness zone should be one in which 100 percent of gardeners can grow the plant without taking any special measures in preparation for cold weather.
I suspect that the people in the lower zones in your example are taking special measures, either consciously (by appying thick mulch covers, for instance) or unconsciously (by situating the plant in a location offering shelter from the effects of cold weather).
