>> When I see growing zones for a plant I see it as a recommendation for hardiness.
I agree. Biology isn't as deterministic as physics. Not even chemistry is as exact as physics.
As you move to the more interesting sciences and engineering areas, rigid or mathematically precise rules become less useful and reasonable interpretations of general guides become necessary. In chemistry, a careful high school student who tries really hard can get closer than 1% to "the exact right answer". But in biology, even in a lab, even one decimal place (+/- 10%) may be doing good! Biology in the field? You're lucky if goats or locusts don't eat your whole experiment.
Despite all the science that has gone into human medicine over the last few hundred years, any given patient with any given disease is likely to display only 50% to 70% of "the" symptoms.
Almost anything said about a plant species or cultivar has to be generalization that varies with many hard-to-measure conditions. Each individual garden is like a new field of study, and the "rules" will be very slightly different in each bed. Observant gardeners will learn either that they can beat "what most people say" about plants' hardiness, or that they have to be more careful than "most people".
