The primary reason is likely to be that hardiness is not a Mendelian trait. Even if you had two Callicarpa dichotoma's with the exact same hardiness characteristics, their offspring might have a very different hardiness. Hardiness characteristics do depend on genetics and are based on the genetics of the parent plants, just not in a simple way like the flower color of Mendel's peas.

How/when/where the hardiness was tested, or even how conservative the rater was, may also play a part, but it's impossible for us to know that without doing the tests ourselves.