Hopefully this graphic will help explain the behavior.
Edit:
The forum software rescaled the image so it's a bit hard to read. Here's the text:
The graphic on the left:
One Below Minimum=1 A.K.A "Preheat"
Light output never goes below the minimum level.
The graphic on the right:
One Below Minimum=0
No light output until a minimum level is reached.
Typical behavior for dimmable fluorescents