Hemispheric climate variation
Another explanation could be axial tilt. If the Axial tilt's rotation is such that it rotates at one safeholdian year and thus always keeping the northern axis pointed away from the star, the North would constantly receive less sunlight than the South.
Even a mild axial tilt would have an effect if it were tidially locked, as I mentioned.
Earth's winters are caused by it's axial tilt more than anything else - however, our axial tilt revolves on the scale of thousands of T-years. With the axis pointed in a fairly consistant direction relative to the sun throughout the year, the sun warms first one hemisphere more and then the other hemisphere relatively consistantly.
A slight axial tilt with a axial revolution close to one planetary revolution would create a condition where one hemisphere would consistantly receive more sunlight than the other for extreme amounts of time. Most likely, the 2 revolutions would not be equal, and the 2 hemisphere's would trade heat dominance, but it could be that entire lifetimes are dominated by one condition or the other.
I don't believe I've suggested that the climate of the northern hemisphere is inherently colder than the climate of the southern hemisphere. I pointed out that climate of the northern and central portions of Haven is very, very cold in the winter; I never said that Armageddon Reef wasn't very, very cold in the winter. For that matter, the average temperature of Safehold is rather cooler than that of Earth, making planetary winters colder, as well, and it has a more pronounced axial tilt, making for greater seasonal variations. Charis, Emerald, Corisande, Zebediah, and Tarot are all on or very near the equator; they have markedly less seasonal variation than the rest of the planet and are always going to be warmer, in both winter and summer, than someplace like Zion. Chisholm has much colder winters than Charis, and the sort of warm current that moderates the climate (somewhat) off West Haven is not part of Chisholm's position.
The biggest reason that the winters are so brutal in Haven (especially) after you get inland from the oceans and north of the Gulf of Dohlar is that you're talking continental winters. Hildermoss Province is 1,650 miles inland and over 3,100 miles north of the equator, whereas Moscow, Russia, is roughly 1,220 miles inland (counting from the North Sea) and 3,846 miles north of the equator. Hildermoss is 1,900 miles north of the Gulf of Mathys; Moscow is roughly 1,000 miles north of the Black Sea. And Safehold is a colder, more mountainous planet than Earth to begin with. The southern stretches of the Republic are as temperate as the southern stretches of Russia, but the inland northern winters are bad, people.
It is true that southern hemisphere winters tend to be less brutal than inland northern winters, but that's because the continents (by and large) are smaller, giving more ocean area to help moderate temperatures. Even so, someone living in the center of Armageddon Reef might not find his toes toasty warm on a winter's night, though.