It means that your doing low carbing right.
The drop is the natural curve of eating and digestion. There would not be a big spike after a low carb meal, so through digestion your body releases insulin to get ready to take care of the glucose In your blood, less glucose, lower blood glucose levels.
It is a lot more complicated of course but that's the layman's description.
It is lower in the evenings because your body is getting ready for your rest and doesn't need your liver to release more insulin at that time after your evening meal, probably the opposite of the dawn phenomenon.
Keep asking questions.