Without trying to be too "new agey", everything changes & moves in a spiral from the massive galaxies in our universe down to the smallest nano - particles & all in between including footballs, branches on a tree, threads on a screw, springs etc. & of course electrons in a wire which is easily viewed on an oscilloscope. A sine wave is simply a two dimensional view of this.
As we are simply part of the universe, why would assume we are any different or our perception of the "sound "of a cable is either. Of course we are not. Thus, cable burn in is as real as anything else & they sound different over time & slowly may get better & eventually worse. Part of this may also be our taste & what sounds pleasing to us which also changes over time & may even come back around again.
Additionally, our ears are amazingly sensitive. From all that I've read in the l've read about loudspeaker development by some of the very best designers, they start out w/ what should sound good in theory in all aspects of the speaker & always tweek their designs by ear over a long period of time w/ different kinds of music. There are no measurement devices that compare to this level of sensitivity & why we can recognize someone immediately over the crappy little speakers in our phones in an instant. Just because something cannot be measured does not mean it's not real.