The reason is that the Maine climate is moderated by the influence of the ocean. The Colorado area is not. For the same reason, an occasional winter in Colorado will be exceptionally warm. Colorado's temperatures are more variable. No matter where you live, there are differences from one place to another over a distance of a few miles, or even a few inches.
Cities tend to be warmer than rural areas.
A planting on the south side of a house and sheltered by evergreens may never get as cold as the plot the exposed north side. It may make a difference that one garden is on a hill, the other in a valley. This is especially true if you are near the southern edge of a zone, or in an urban area. Heat is important. We tend to think of plant survival in terms of cold survivability.