The hardiness zones are set at 10 degree F intervals. Zone 10 is between 30F and 40F, zone 11 is between 40F and 50F. If you're only just in zone 11 then you're not much different to where it's the top of zone 10.
Remember, this cold weather will go into the records and be used to calculate zones. It's based on average lowest temperatures. It's a statistical calculation.
The zone system is calculated and set by the USDA. They use the data from weather stations around the USA. I don't know where "Christiansted, VI" fits in or where it actually is, you'd have to go back through the official meteorological records there to be able to make the calculations. For the USA the USDA has already done it. But it's not fool-proof, there are microclimates near weather stations which could be above or below what the official records state. It's still an imperfect guide, but the best they've been able to come up with. Other parts of the world are adopting the same scale for consistency. Nothing gets below zone 1, and it's easy to add numbers at the other end.
There are all sorts of micro climates on this island, dry and cactus on the east end, the wind comes from the east, and very wet and green on the west side. Salty in a lot of places, and cool at higher elevations. Very interesting.
Just no chilling for berries and fruits like apples and pears and raspberries. I did see one grapevine on an island 400 miles south of here, Bonaire. It may have been a Muscadine.
That system is based on coldness. There's another system called "Heat Zone" based, of course, on how hot a place gets (consistantly hot). It's not used much as most people are more worried about getting plants 'fried' by cold rather than fried by heat. If you're on an island the temperatures are moderated, you don't get so low in the cold zone and not so high in the heat zone.
I searched on the weather underground site for Christiansted, VI
. Not a lot on their records. You're further from the equator than us, but we're not an island. Seems you have consistently less rain right through the year, we have much more rain only for half the year, no rain the other half. Not having high mountains means your rain blows right past most of the time. High mountains would force the humid air up to higher, colder altitudes and cause precipitation. However, atmospheric vortexes with the help of warm seas will lift moisture into higher altitudes (possibly form cyclones) and also give you precipitation. Similar happens here, but because of our high temperatures sea breezes are also drawn inland and up into the higher altitudes causing storms which eventually make their way back to the coast. Weather is a fascinating subject.