There are also reflections and cancellations that you'll see just like ripples in a pond. This is elementary physics at best and advanced RF theory at worst.
In short. Distance is going to be a factor along with reflections, what the signals have to pass through and competing signals. At close range most of these factors disappear.
So all is not goodness and light in that small space. If you place your devices just so in a dead spot you get nothing or much less performance.
I noticed that when i'm just 2feet away from my router, i get my full 50mbps up/down and only get 20mbps when i'm approx 10ft away behind a 2-Inch thick wall.
From my understanding(pls correct me if im wrong), as long as your wireless device is tethered from the AP, then speedtest sites would measure the speed and bandwidth of your service. So why would the speed and bandwidth degrade when my wireless device is farther?