As per the disclaimer on the Coverage Map;

"These Coverage Locator maps depict predicted and approximate wireless coverage. The coverage areas shown do not guarantee service availability, and may include locations with limited or no coverage. Even within a coverage area, there are many factors, including customer’s equipment, terrain, proximity to or inside buildings, foliage, and weather that may impact service. Some of the Coverage Areas include networks run by other carriers, the coverage depicted is based on their information and public sources, and we cannot ensure its accuracy.