Gardening maps that divide the country into hardiness zones are revised periodically, and boundaries have generally moved. The maps describe one narrow statistic, and knowing which one explains what a revision does and does not mean.
What the zones actually measure
A hardiness zone is defined by the average annual extreme minimum temperature, meaning the coldest night in a typical year, averaged over a long run of years.
It is not average winter temperature, snowfall, or length of cold season. A single number describing the annual low is the entire basis.
Zones are divided into bands of equal width, which is why a modest shift in the underlying average can move a location across a boundary.
Why that statistic was chosen
Perennial plants tend to die from a single hard freeze rather than from accumulated cold, so survival depends on the worst night rather than the seasonal mean.
A woody plant that tolerates a long cool winter can still be killed outright by one deep freeze, and the zone system encodes that asymmetry.
The design makes the map useful for its intended purpose and misleading for almost every other one.
How a revision changes the picture
A new map combines a fresh block of station records, so extreme lows from earlier decades leave the calculation and recent ones enter.
Because the coldest nights have moderated across much of the country, the recalculated averages rise and many locations shift a half zone or a full zone warmer.
Denser station coverage and improved interpolation between stations also change boundaries, particularly in mountainous terrain where a single station once stood in for a wide area.
Why a warmer zone does not guarantee survival
The zone is an average of annual extremes, so roughly half of years produce a colder night than the zone value implies.
A plant sited at the edge of its tolerance survives ordinary winters and is lost in the unusual one, which arrives on no schedule.
Experienced growers therefore treat the map as a filter for what is plausible rather than a promise about any given winter.
What the map leaves out entirely
Summer heat is absent from the system, so two locations sharing a zone can be unsuitable for the same plant for opposite reasons.
Soil drainage, humidity, wind exposure and the timing of the last spring freeze all affect survival and appear nowhere in the calculation.
Late frosts matter especially, because a warm spell that breaks dormancy early leaves buds exposed to a freeze that the annual minimum statistic never registers.