Pete wrote:
"So now my logical question is why are the FM whip antennas even
extendible if they play better fully collapsed?"

Length is adjustable to tune the antenna to resonance so that excess
reactance does not block too much signal from the receiver. Resonance
depends on the station you want to receive.

Wavelength = 300 / MHz

At 100 MHz, wavelength = 3 meters

If the best antenna length were 1/4 wavelength, length would be about
0.75 meter = 75cm/2.54cm/in.= 29.53in. which may be shortened by 5% for
"end effects" which calculates to about 28 inches for a 100 MHz station.
Higher frequency stations may require shorter lengths and lower
frequency stations may require longer lengths. We don`t know the design
of the radio and what inherent reactance may reside inside. So, we
really don`t know how long the adjustable antenna needs to be.
It is far better to experiment with the length of the antenna to get the
best results.

Some radios use the same antenna for several bands of a multiband tadio.
So adjust the antenna if necessary for best results.

Best regards, Richard Harrison, KB5WZI