If I had to take a guess I would say that permanent magnet synchronous generators (PMG) operate at a higher frequency than the generator output (50/60Hz) as they are operating at a synchronous speed off the rotating generator that they are mechanically coupled to.

Just a guess mind you.

guest; it makes for smaller transformers, less ripple if rectified with smaller capacitors for a very smooth DC out put, the military used 400 HZ, & 800 HZ for smaller & lighter equipment. perry

Higher the frequency, the peaks occur more often, lesat breaks between peaks, almost resembling DC, making rectification easy.