The USDA zones only assess one of many variables that determine what can be successfully grown in a particular climate. That is, the average annual minimum temperature for a given location, based on the most recent 30 years of data. For example, downtown Orlando and the innermost suburbs are now a zone 10a (average annual lowest temp 30F-35F) due to the urban heat island effect. In the past (say, before 2000, give or take), there is no way Orlando would have been considered a zone 10, as temperatures in the 20s occurred in most years multiple times. This has little, if any effect on how I practice and handle my bonsai, as the tropical species are going in the greenhouse when lows are below 50F, just to be safe (so whether the low is 27F or 34F, it makes no difference to them). The Japanese maples, sugar maples, Korean hornbeam, trident, etc. are going in the fridge for 3-4 months regardless of whether my location's zone is a 9b or 10a.
The city of Fort Myers (for example), in S. Florida is also a zone 10a, but averages a good bit warmer than Orlando in the cool season, especially during the day, due to the location's lower latitude. As a result, tropicals will receive less winter stress/damage than in Orlando. Conversely, some temperate species that are at their southernmost limit in the Orlando area would not prosper as far south as Fort Myers. Whether the temperate species (which have been native to the area since WAY before mankind ever arrived) in the Orlando area are affected negatively by our urban heat island bumping up low temperatures on the coldest nights by ~4-7F degrees, I have no idea, but I suspect any effect is minimal at most.
I have a yard full of tender palms (including 15 coconut palms) and other tropicals, many/most of which I will lose to a freeze in the very long term, but in the "old days", things like coconuts would be lucky to survive a year or two here. Not the case anymore. Go to the rural areas well outside of the city, and it's a totally different story.
I have done quite a bit of research into this subject, especially as it pertains to my locale and to the State of Florida. There are pockets of the NW FL panhandle (well inland) that are zone 8a, while the urban core of Atlanta borders on (or in a few spots actually is) an 8b zone. Downtown DC is now a zone 8a (yes, there are some winters that get below 10F in downtown DC, but the 30-year average is between 10-15F). So you compare a zone 8a spot in NW FL to DC, and the average annual minima may be in the same category, but the average temperatures are worlds apart. January is 37F low / 62F high in the 8a NW FL and 27F low / 43F high in 8a DC.
I have read anecdotally that there are locations in the southern Outer Banks of Northern Carolina that are zone 9a (average lowest annual temperature 20-25F). I haven't done enough research to see whether it is likely true or not, but I certainly believe it.
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