It's important to remember that the average first fall frost date is exactly that: an average. It varies from year to year, depending on the season, and can swing wildly.
Microclimates also have an effect. Your yard -- or a spot in it -- may experience frost well before other areas in your neighborhood based on geographical factors such as low spots where cool air pools at night. Likewise, a particularly sheltered spot may not have frost until several weeks after other areas.