Don, Jack is definitely correct with regard to 5a being colder than 5b (just think about the overall zone thing... warmer is higher numbers)
But there are so many microclimates that the zones really can't address. And then a few years ago the USDA revised the map and moved a lot of us to a warmer zone than we had previously been in, which, in my opinion, is just so much "horse hockey" in the words of the Colonel on MASH. They can say I'm in zone 5a all they want, until we get a winter like we had a couple of years ago... then, 4b might be a stretch.
