Just don't confuse planting zone with last or first frost.
I live in zone 7a (Philly) but have a bit of a micro climate from the Delaware river, my mom is in the same zone, 4 miles south of me and there is a 7 day difference in last and first frost dates.
Based on zip codes we are the same last and first frost, however I am on the outskirts of my zip code and technically closer to a cooler zipcode.
Head another four miles south from me (Bridesburg) and there is a 9 day difference, head even further south to the Philadelphia International airport and there is a 6 day difference from my area.
Micro-climates can be a bit tricky.
"Do not trust any planting zone frost date chart which lists last frost date or date ranges per zone, as last frost date and zones are not specifically correlated. Planting zones are defined by minimum extreme temperatures, while last frost date depends on specific regional and micro-climate environments. Especially in today’s world of new climate extremes, you must pay attention to local indicators and weather forecasts and use frost protection strategies if you plan to push the envelope".