To tell you the truth, cold hardiness is far from an exact science. Micro climatic conditions come into play. The type of rootstock used can also make a difference. One thing is certain, the healthier you maintain your trees during the entire year, the better they will survive a cold snap. You have to take many factors into account when you judge how cold tolerant a tree really is. I grow my most delicate trees in a protected area on the south side of my property. One example of cold hardiness that I found during the cold snap we had in SFLA in 2009 was that my grafted J-31 Jakfruit did much worse than my seedling Jak. My J-31 lost 60% of it's leaves and had branch damage. It's on the protected side of my house, whereas my seedling Dang Suria Jak is in an open section of my yard and suffered zero damage. Before you judge how cold hardy something is, you need to examine all the evidence and over several years.