I would suspect it is a combination of two things. One is that these little "horn" multi-purpose antennas they use on the back of the roof are not optimized for all frequencies since they must deal with Sirius, FM, AM, and satellite data all in one unit.
Second, FM is subject to multipath distortion from reflected signals. If you drive through town with tall buildings or beside a semi truck on the highway, you get reflections that are slightly out of phase with the main signal. Those out-of-phase signals effectively cancel the signal until you are past the reflective surface. Same principle used by noise-cancelling headphones in using out of phase sound.