The map has an explanation:

About this data

The 2012 USDA hardiness zones were calculated using the average lowest winter temperature for the observation period of 1976-2005. The new zones are calculated using the years 1991-2020. These two observation windows overlap. Colors show the difference between the two 30-year averages for each place on the map.

My area is 3.8F warmer using this method.

FWIW they likely mean Fahrenheit. I.e. 2.5 Celsius.

Yes, I did. 4.5 degrees Celsius would be a newsworthy increase in winter temperatures! :)

Where I am at, in the Canadian Rockies, the daily temperature this winter was routinely 7-9 degrees Celsius above average (1970-2020) according to my iOS Weather app. They introduced this feature last year and I have been checking it nearly every day. Other than a cold snap in January (that killed my bees!) the weather was warmer every single day.

You're hitting on another issue we gardeners are noticing - overall the temps are warmer, causing things to bud early, but there are still cold snaps. The cold snaps aren't any colder than they should be for the time of year, the problem is the inter-snap days are a lot warmer. This increased variance in temperature is actually worse than the average temperature getting warmer.