The frequency response of your audio system is different at every position
in your room. This is caused by the vector addition of sounds from two or more loudspeakers, each producing a different phase shift at every different frequency because of different distances from the listener. The same thing happens when sound reflects from a room surface or object in the room. If you move your head just a few inches, you
may hear a noticeable change in sound quality.
Tests for the audibility of tweaks or changes of equipment may result in false positives if you or anything else moves during a test. More details and some sine wave measurements
appear in an article in a new book: