FBS detectors utilize a high freq of about 25 kHz and a low freq of about 3.125 kHz. The other advertised frequencies are just harmonics of the low freq and aren't used at the receive coil, though Minelab is correct in saying they are transmitted, there are many weak harmonic freqs transmitted - but they aren't used for any purpose.
The Noise Cancel feature shifts the frequency slightly up or down. The normal high freq of 25 kHz,which would normally be Channel 6 would be pushed to 27.78 kHz if Channel 11 were selected or dropped to 22.2 kHz if Channel 1 were used. It is enough to move the detector away from interfering radio bands - but not enough to make much of any difference in sensitivity (though some would argue it does).
The rest of the miracle of FBS technology is how it analyzes the X and R components of the received signal (and ground signal). It is like a PI in a sense in that it uses the time constant of the target signal to measure the signal components at points along the decay curve and then compare them. It's pretty clever.