Hubs rotate at a much lower frequency than bottles, and though they have more poles to somewhat compensate, they still end up delivering a much lower frequency. At 15 km/h, the B&M Dymotec6 bottle dynamo (8 poles) outputs 168 Hz AC while the Shimano DH-3D71 hub dynamo (28 poles) outputs only 28 Hz at the same speed. At 8 km/h, a 28 pole hub is down to 15 Hz, which causes visible flicker of the light.
http://pilom.com/BicycleElectronics/HubDynamo.htm