if not smart group, it is just that for whatever reason, the other device actually heard you and responded. Sometimes when I am standing next to one in the dining area, a bedroom one will respond. It just happens.
Though I am not a sound expert, I believe it has to do with sound waves and the way it moves throughout the house because of other noise, ceiling fans, where you are facing when giving the command and more. It also has to do with the device itself as to how fast it picked up the sound with the microphone. Older devices reacts slower than newer devices. When multiple devices hear, ESP kicks in and the first one who receives the command responds. ESP is an echo feature where only one device will respond no matter how many heard.
The only solution is to move the device away line of site to the door in the back rooms, put further from the door on the same wall as the door. Also lower your voice a bit when giving commands.