It's important to remember that the average last spring 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 after 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 stop seeing frosts several weeks before other areas.