Ask Mr. Smarty Plants

The USDA Plant Hardiness Zone map says that it is based on "the average annual minimum winter temperature, divided into 10-degree F zones."  It doesn't really address other important factors in plant hardiness, e.g., the heat, soil type, or rainfall of a particular area.  One possible reason for the multiple zones in a plant's description is that according to the article, Maps and Gardening, from the USDA:

"Hardiness zones are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future."

Additionally, there can be microclimates within zones where the colder temperatures go below the annual minimum.

This article (Maps and Gardening) addresses the other factors affecting plant hardiness.  Probably your best guide for whether a plant will thrive in the summer temperatures is to look at the environment and climate of where a plant grows naturally.  On the USDA Plants Database, you can find maps showing the areas where plants have been found growing as natives or as introduced species that have become naturalized.  You can search by scientific name or by common name for your plant of interest, but your best bet is to use the scientific name since common names vary from place to place and the USDA lists only one common name per species.