Hardy Palms In Temperate Zones Board : Uh oh, More Unexpected Hardiness

My winter low for the season, was -10F. This temp is the lower limit of z6. Yet, these two trees that are common in the subtropical zones, came through this past winter, with flying colors, leafing out to the very tips of their stems....UNPROTECTED!

The first is a late leaf-budder. Some of you can probably understand why I said that we should not be too impatient, and cut everything down, too early. Some things just naturally break dormancy later. The tree in the pic is a Parasol Tree, Firmiana simplex, I got from Brian's Botanicals, in Kentucky, about 4 yrs ago, in a 4 inch pot. I planted it in the ground, expecting to have to protect it every year (thank goodness, it is a slower grower, and would not quickly get "too tall to protect"). In any event, since it was rated just z7 hardy, I was able to protect it, with just a styrofoam rose cone, its first winter. The second winter, I used a taller, 30 gallon Rubbermaid garbage can, to cover it with. However, for the past two winters, I used no protection, at all. Notice the date on the calendar, that it is just breaking bud:

https://photos.app.goo.gl/9oF4JtJENyvxmSra7

The tree below is a Dove Tree, Davidia involucrata 'Sonoma', I got from Gossler Farm Nursery, in z8 Oregon, last year, as a 5 gallon. It too, experienced -10 F, in great shape:

https://photos.app.goo.gl/JtCCSfNam1FvJs8r6