What's interesting to me is that one particular station came in clearly, given how the difference in broadcast FM allocated frequencies translates into millimeters of antenna length. (e.g. 98.3 vs 98.7 MHz = 3051mm vs. 3039mm)

Just some flexing / coiling of the cord could mean you would favor a different station or get tons of mixing in of other stations.

Maybe the key is that the author / friend was very close to a broadcasting antenna and that overwhelmed any other station that would equally have been received on the headphone cord otherwise?