Hardiness of plants depends on many factors. Next temperature moist and wind are very important factors that determine if plants will survive the winter or not.
The information I have given on my website is from my own experience and also information I have found in the internet. If the internet gives too much contradicting information I prefer my own experience. If the information on the internet is the same everywhere then I use that on my website. It is important that you know that I have a very sheltered garden. There is not much wind and the soil is sandy and well-drained and there is no salt in the air.
If you have clay in your garden then some of the same plants will grow differently than in my garden. The same goes for a garden with a lot of wind. You may have to shelter some plants against that wind or otherwise they might freeze and die.
Wet winters may have deadly consequences because your plants may rot and if that moist freezes the growing centre can be damaged. Palms can be quite susceptible to this. It helps if you cover the growing centre against precipitation. Do not leave that cover there the whole winter because the plant may rot.
It also matter for how long it freezes. With one or two days of frost the ground is not yet totally frozen. The chance of plants freezing to death then is not as big as when the frost lingers on for a longer period.
Often the hardiness is expressed in terms of zones. Those zones are based on the average minimum temperatures that are measured in those zones. Unfortunately that information is not the same everywhere and as I wrote above there are more factors than just the temperature to take into account. The zones and their temperatures are not enough to determine if your plants will survive the winter but it gives an indication. If a plant prefers a dry to normal soil then it is possible that that plant will not survive a wet, cold winter.
If I like a plant I put it in my garden. Based on the information about the ideal situation for the plant I choose a position. Then I will find out how the plant does and if it survives the winter. Very often the information has been incorrect or insufficient. I use my experience and add that to the information about the plants on my website.