There is a problem with the magnetometer in iOS 7, it has an error of +-7º. Try using the 7.1 beta version.
EDIT
The magnetometer has zero-drift over time, but is pretty inaccurate for sudden changes in position. The accelerometer and gyroscope on the other hand adjust quickly for sudden changes but, being inertial sensors, they lose accuracy over a period of time.
So when CMCalibratedMagneticField
tries compensate for your rotational motion it uses data from the gyroscope and accelerometer. This is when the accelerometer and gyroscope's +-7º error creeps in and throws your circle off track. Check this answer and this wikipedia article for more info.
As regards to the figure of eight: Both do the same thing, they orient the "North" of your device in each direction in hope of cancelling out magnetic interference. Flipping your device along all three axes will work better but it is harder to perform and not as easily understood by the user.
Hope this helps.