I think you're probably on to something since when I plug in my ZIP code the USDA map says I'm in Z5b but the U.S. National Arboretum map and GardenWeb both put me in Z6a. Typically we see a few nights when the temp dips down to minus 9 or 10, maybe even minus 12 once in a green moon but it isn't the AVERAGE winter low.

My neighbor three houses to the north insists his garden is Z5b but he's going by the fact it gets far enough below zero once a year to hit the Z5b low temp.

Someone on the Winter Sowing forum posted a link to a site called PlantMaps and that one has a zoom feature so you can punch in your ZIP code and get a close-up view of the USDA map. Naturally, when I zoomed in on my ZIP code, it showed me in Z5b.

I'm also going by a few Z6 plants that self sow in my garden. If I'm Z5b, they shouldn't come back each year but not only do they come back, they're growing in the cracks in my concrete breezeway.