Hardiness zones are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future.

Plant hardines zone maps are just guides. They are based on the average lowest temperatures, not the lowest ever. Gardeners should keep that in mind when selecting plants, especially if they choose to "push" their hardiness zone by growing plants not rated for their zone. Also, keep in mind that there might still be microclimates that are too small to show up on the hardiness zone map.