[rcxdude]

My induction hob does similar on low power settings (at the lowest it's something like 1 second on to 4 seconds off). On higher power settings it's not noticable. I suspect the main reason for this is it's often harder (and more expensive) to design power electronics to operate over a wide PWM duty cycle efficiently, so the low-speed cycling is a way to provide the low settings without a significant cost increase.