'Frosty' Canadian hemlock is not named in honour of the snowman, but rather for the white colour of the new needles on the tips of the branches. As you can tell, this phenomenon is not well-displayed by this photograph. Instead, the focus is on the cones and the shadows from winter's low sunlight.
I have the impression that the majority of conifer cultivars originate as a selection of a physical variant that is somehow different from a typical plant in the species, but if someone with more knowledge wants to qualify or dispute that assertion, please add a comment. As a comparison, though, you might recall from the entry on Gladiolus flanaganii that the thirty thousand cultivars of Gladiolus are virtually all due to hybridization and breeding.
Since I suspect that conifers might be on your mind this month, you might like to spend a little bit of time browsing around the site of the American Conifer Society or browse through the garden's conifer discussion forums.
Botany resource link: “hamadryad”, a member of the garden's discussion forums, submitted this article about a study from the University of Washington – “Wildflower seed mixes include some wicked bloomers”. From the article: “When growing 19 such packets of wildflower mixes, however, University of Washington researchers found that each contained from three to 13 invasive species and eight had seeds for plants considered noxious weeds in at least one U.S. state or Canadian province.”
I'd guess that has a lot to do with generation time - much the same can be said of cultivars of most other trees. When there's a 10-30 year generation gap between first and second generation hybrids, not much gets done.
Tsuga canadensis - Z4, RHS Index of Garden Plants, Griffiths
Conifers are wind-pollinated. To cross them you'd have to do something like cut a branch of one and shake it next to another, that had been kept enclosed in a bag to prevent uncontrolled pollination. I think this is the kind of thing that is done to select for timber production, but apparently not so much in ornamental hort.
Welch & Haddow (1993) attribute name 'Frosty' to Welch, via Swartley's 1984 hemlock book. W & H also say it's a bush. Gelderen & Hoey Smith (1996) call it a "medium-sized tree", say it's a "good grower."