Yup, the spin is measured as the cursor's angle around the center of the screen, so the radius of your circle quietly matters. If you spiral inward without realizing it, the cursor drifts toward the middle, the angle stops changing much, and there's a small dead-zone right at the center, so the spin can die on you with no obvious feedback. That center-dependence is exactly the bias you're feeling.
Thank you for the feedback and for playing!
Good luck with your voting :)