AFAIK
- never :
If you zoom manually, it will stay at that zoom-level - fixed :
If you zoom manually, after some time it will change back to the fixed zoom-level.
(Independent of distance to next turning point) - automatic:
Automatically changes the zoom-level dependent on the distance to the next turning point
(zoom in when approaching turning point, zoom out otherwise)
Yes, this is missing for automatic mode. 
Manfred