FYI the measurements used by the USDA are Fahrenheit, a zone is +/- 10F. There are zone 1-13 though not all exist in the US. If I recall correctly about a decade or two ago we began subdividing them into A and B zones. Where before you had zone 6 for 0 to -10 (my local zone) but now you have 6a (-10 to -5) and 6b (-5 to 0).

Its also not just the average minimum temp, its the average minimum temp over the last thirty years. A few years ago my zone changed from 6a to 6b following the regular reevaluations the USDA does.

Edit: the A/B zones were developed in 1990