Dithering Modulation Frequency and Dithering Rate
This is basically FM modulation of the carrier PWM frequency.
The usefulness is not so much in performance with crosstalk to other tuned circuits but rather to reduce the quasi-peak amplitude of unintended radiation.
Here they chose an FM BW of 10% pp approx and 2% modulation rate or an FM ratio = 10/2=5 to reduce the quasi-peak EMI possibly radiated or conduction on the AC grid.
Let me see if I can simulate this from FCC or EU EMI setups for quasi-peak BW which were derived from old days of AM radio interference with an RC filter on the peak detector.  It might not make much difference since 10% here is still audible if converted from FM to AM to baseband audible noise for this LF radio band which might only be 4kHz.
In PC BIOS setups some have spread-spectrum options to do the same on bus clocks used for CPU clock for the same reason but in higher Radio bands.