There is no reason that the motor axle and motor case should be spinning at the same speed- they aren't connected after all, and it's pretty obvious when you watch the motor operating normally since the case just sits there and the shaft spins!
When you say it as spins brightly as before, that is probably with a very weak, very fast motor when slippage is the most. If you try it with a high torque, low speed motor it should be less bright since the motor is able to move it's own case more easily resulting in less slip.
I'm a bit curious how you actually found this out since wouldn't a motor spinning at 20,000 RPM wind up the wires between motor and light bulb really fast?