Just because inside of the hub is a circle of smaller individual magnets I would think. As the wheel spins it will not be entirely smooth as each magnet magnetic field interacts with the induction wiring (I'm guessing that's how they work). Making a slightly notchy feel which would be felt as vibration in the bars. Though I haven't felt that when I've (briefly) ridden a dyno hub.
I can feel the notchiness when spinning a wheel by hand but in no way have I ever noticed it while riding. Perhaps he's spoiled with Dutch paths. Over here you need tank tracks just to get to work...
Just because inside of the hub is a circle of smaller individual magnets I would think. As the wheel spins it will not be entirely smooth as each magnet magnetic field interacts with the induction wiring (I'm guessing that's how they work). Making a slightly notchy feel which would be felt as vibration in the bars. Though I haven't felt that when I've (briefly) ridden a dyno hub.