Just out of curiosity I entered my zip code in the zone finder and it came up 8a. I looked at the map and it shows 8b. My zip code does not extend very far and all of it is clearly within the 8b section on the map. (Actually much of the area is really 9a due to being small micro-climates of sea islands surrounded by warm salt water.)

I know the map is as correct as it can be, how come the zip code finder is off???