Think of it as an AM radio station signal. You can get better reception by standing next to a window, or by raising it up over your head, than you can if your car is in a tunnel. Your wireless broadcaster is (almost) the same thing as an AM Radio station, broadcasting for you.
For a non-technical answer - If you’ve placed the router in an enclosed room in the center of your house, the radio signal has to try and penetrate through the walls and/or studs and/or metal studs in the room in order to get any distance. If it is in a room with a window, the signal is going to be able to get out the thin sheet of glass incredibly easy. If you are downstairs trying to receive the signal, and you’re near a window, the signal going through the window will get much less interference through the two windows than through the two walls, concrete floors, etc.
One thing to check in your house - If you are losing signal at 20 feet, are you seeing broadcasts from surrounding areas? Sometimes your laptop might be trying to latch onto that signal, rather than keep in contact with yours.