Yeah, the usda zone system is based on expected minimum winter temperature....
https://planthardiness.ars.usda.gov/ My zone zone 6a should expect cold of -5 to -10 F, and a 5a zone should expect cold between - 15 and - 20 F. Without knowing your exact location, I'd surmise my micro climate is warmer then yours because I'm basically between 3 of the great lakes and large bodies of water moderates the local climate, keeping it slightly warmer in the winter and slightly cooler in the summer. This was my first winter here and the coldest it got in my back yard was - 6 F... so far, so good! Supposed to get 1-3 inches of snow tomorrow... yay Spring!!