Why is the altimeter unwinding unevenely during a spin?
The altimeter works by measurements of the pressure in undisturbed air, also called static pressure. A static port is placed, usually on the side of the hull, perpendicular to the airflow. 

During a spin, the airflow will not flow evenly past the static port, but will occasionally flow directly onto the static port. This will cause a rise in measured pressure, leading the altimeter to believe it is lower than it actually is. This cause fluctuations in the indicated altitude.