As a rule, nurseries or plant sellers hedge on their hardiness claims, with some exceptions, of course. If a plant is known to do well in one USDA zone, it will often be listed as recommended for the next warmer zone. This is a safety net for the retailer who will have to replace fewer plants that didn't grow.
Another method of exacting a hardiness zone has to do with where it was developed. For example, a few years ago, a New York Nursery introduced a new nectarine for zone 4. So I sought out the person who developed it asking, "Will it thrive in zone 3?" He hadn't tried growing it in zone 3. I do a similar thing with the Pequot Black Raspberry that I sell. I developed it in zone 3 and recommend it for zone 3. I do have some plants thriving in zone 2 but I'll not recommend for zone 2 because unmulched plants are not likely to survive a snowless, -50° winter.