The higher number is the upper limit. Not all plants can handle tropical conditions and need a winter rest. There are several plants I can't grow here in south AL that were common to me in OH. Lilacs (Syringa,) tulips and Hyacinth bulbs come to mind readily. There's not enough chill hours for the buds to form, which happens during winter rest/dormancy. These plants common to a central OH landscape are nowhere to be found near the FL border. If one wants a tulip or Hyacinth to re-bloom here, it must be artificially chilled.
There are other plants that just don't tolerate high temps, but I can't offer a specific, personal anecdote. Plants called alpine are a good place to start an investigation though, as they are generally averse to high humidity, low elevations, and high temps.
