Nah, most likely, your garden is warmer simply because it's close to your house. My yard covers 4 zones, namely, 7a, 6b, 6a, 5 and 4, staring from the cold frame by the southern wall. My veg garden is mostly 6, because we don't want it too close to the house, where we use a lot of pesticide, nor on the border, where there is car exhaust or the run-off from neighbors' lawn.