AM stands for Amplitude Modulation. FM stands for Frequency Modulation. Generally, there is less inteference on FM than AM. As proof, listen to your radio in your car. Tune an AM station and listen to the static, especially distant stations. Then listen to an FM station. Much less, if no inteference, especially when there is a storm in the vicinity. As the receiver gets further away from the transmitter, the signal begins to get more static and would be more subject to inteference than a similar FM radio. That's mainly why people have gone to FM over AM. That may be the case with the radio you bought. Someone out there upgraded to FM and wanted to get rid of his/her old AM set. You were the 'lucky' one.