Just looking at weatherunderground for your location, it shows the temps for your city dipping down into the mid 30's early this January, and even colder temps last January. It may not happen often that the temps drop that low, but the hardiness zone is accounting for the worst case scenario.
That said, hardiness zones really only tell you about your macroclimate, and in a coastal region like you're in, that can be deceiving. It could very well be that since you are so close to the ocean, your yard never does see the low temps that someone living a few miles inland would. It is very possible that you could get away with planting something that should be a half a zone (or possibly even a full zone) warmer than what your hardiness zone says.
My yard is in zone 4B, but I have a spot on the south side of my house where I can get away with 5B plants most years. I say most years, because we had a terribly cold winter this year and my garden has yet to wake up - I won't be sure for another month whether the marginal plants made it through this time.