loudness 1
First, how FM works! FM is Frequency Modulation, the frequency transmitted varies around the centre carrier frequency (e.g. 98 MHz) that you set on your radio — it would stay at exactly that frequency if the station transmitted silence — and it deviates proportional to the instantaneous value of the audio signal transmitted.
The stations want to appear louder , so the audio is processed, by compressing it's dynamic range (the range from quiet to full blast) which makes the quiet parts louder *without* increasing the level of the loud bits beyond the limit that would cause over-deviation and earn a fine.