Obviously there is still light in the sky, there is a transitional period...and the way your eyes utilize that light is different from how a plant does....once that light drops below a certain level (I'm sure you could research light threshold in cannabis plants and find out exactly), probably nautical twilight, the intensity is too weak and the plants can begin their dark period...then the opposite takes place in the morning. The whole point of switching to 12/12 is to quicken that transitional period in the plant artificially and immediately put it into darkness. Virtually everything in regards to growing inside is going to be different than nature. That is the point. To control every aspect. If you want to set your lights to do this gradual transitional change and prolong your grow, go right ahead. The hard 12/12 just eliminates this transition - but if you have LEDs in there, or it is near a window with street lights or light from under a door coming though, that intensity is too great for the plant, even though it looks weak to you....you can see evidence of this on this site all the time. Someone is always posting a pic of their plant in a tent with LEDs causing it to reveg.
Sunlight intensity outdoors is different from a desk lamp intensity indoors obviously, but just because twilight is weak for a plant does not mean a desk lamp is weak for a plant as well.