Personally, I wouldn't dare leave a potted black pine out exposed to temperatures lower than the low 20s, as I've always heard they are quite cold sensitive. I would think a juniper would have a better chance of making it so maybe it already did have problems going into the winter.
As others have noted, there are many factors at play other than the actual low temperature. For instance, how long was the temperature down near 13F? Did it drop quickly into the teens and stay there most of the night, or did the temperature plummet down for only an hour or two? Remember that the air will cool much faster than the soil in the center of the root mass, so if it was 13 only for an hour or two, the bulk of the root mass would have not reached anywhere near that temperature (unless it was windy).