Location, location, location.....
Because simulcast is basically a number of individual towers all using the same control channel (frequency), your radio is receiving THE SAME INFORMATION from multiple towers simultaneously.
Example: If one signal is straight line of sight and another bounces off of a couple of hills before getting to the radio, the signal bouncing took slightly longer to arrive - and thus is slightly delayed. This causes multipath distortion. Some radios handle it better than others. Or... the physical position of the radio allows only one of these paths...
Another solution (other than a 100/200) is to use directional antennas, so you are 'omitting' the bounce(s)...