It's important to differentiate between the speed of the Internet connection 'to your home' and the speed that reaches an individual device 'within your home' because they're two very different measurements.
Internet speed tests, like this one or the test found at SpeedTest.net, measure the latter, or the speed reaching the device running the test. These test results are often lower than your plan speed due to various factors outside your Internet provider's control, including WiFi conditions and device capabilities.
Good coverage means your device is getting a strong WiFi signal. That strong signal will also deliver as much speed as your device can handle. As you move away from your Gateway (modem or router), signal strength diminishes, slowing upload/download speeds and reducing coverage. The placement of your Gateway is the key to ensuring both coverage and speed for your devices.
Your home WiFi connection is only as good as your Gateway (modem or router), since all devices connect through it. We upgrade our smartphones regularly, but often neglect to upgrade the actual devices that connect us to the Internet. A Gateway that's a generation or two behind may not be able to deliver the fastest broadband speeds to the latest devices. If you have an older Gateway (modem or router) consider an upgrade.
Different devices handle speeds differently depending on make, model and age. Consider the speed of a race car compared to a minivan. You wouldn't expect the same top speeds, and the same is true of your devices.
The Internet speed delivered to your home is shared among all your devices. If there are too many devices connected and using the Internet at once, there may not be enough speed to go around. The result will be a slow-down across all your devices.