Bandwidth and shift are two separate things
Hopefully this is accurate. I'm an EE, but have never worked on RC systems other than as a hobby. If I'm wrong, I'm sure someone one RCU will jump in and correct me ;>).
The bandwidth is the deviation in frequency from the "center" frequency cause by modulation. Transmitters (simply speaking) transmit around the center frequency called the carrier. This is what channel you are on and the receiver filters out anything outside of the bandwidth. Single conversion receivers do it once and double conversion receivers do it twice and therefore reject a greater amount of interference. The slight change in frequency is then demodulated into the pulse trains seen elsewhere in this thread.
The joystick (servo) signals in the transmitter are modulated onto the carrier frequency and each channel has a period of time that is allocated to it in the pulse train, then it starts over again. The pulse can vary from 0-100% of this time period and that corresponds to the servo throw from end point to endpoint, however, one type of shift has the pulses going low for this time and the other has the pulses going high for that same time. The reason that they just don't reverse the servo throw is that the time relationship must be reference to the start of the servo signals and if the shift is reversed, the receiver picks the wrong time period for the servo signal.
Some receivers now can switch the waveform and switch their shift to allow use with both shifts. From an engineering standpoint the shift change is easy to put into a transmitter or receiver when it is designed, but would not be practical to retrofit.