[Karlos]

FM synthesis, or what most people think of when they hear the term, is actually phase modulation. The instantaneous phase of the carrier wave is modified by the modulator. There's no actual change of frequency, this is the emergent property of the phase modulation. The phase adjustment is linear - take the instantaneous amplitude of the modulator, multiply it by some modulation index (intensity) and add it to the phase input for the carrier. Both carrier and modulator are both operating at some fixed sample period in a digital implementation.

What Paula offers is Period Modulation. The period register for a channel can be fetched by a different channel and poked in. The two channels are not operating at the same period, unless by coincidence. The whole arrangement is very different, but it is a genuine hardware feature.