It's because anything within .1 frequency will be picked up by other channels. Notice everything ends in a .1 .3 .5 .7 or .9
Radio waves can only carry a certain frequency. The wavelength suggests the quality of the sound. FM radio is made for music. AM Radio is made for talk shows n' stuff.
There are only a certain number of wavelengths available before the channels start blending.
Try it on an AM Station. Go exactly halfway between two channels and you can hear them both (amongst static).
Edit: Pretty sure that AM may have a different suffix for its channels.