Hi
@stepside ,
The Cal/Mag is dropped to ensure cleanliness of your final product. With added nitrogen and magnesium, using MagNifiCal up until the end of flower can lead to unfavorable flavors while smoking.
That being said, people who grow for extracts typically aren't concerned about the quality of burn (because they are extracting only the resin) and have a tendency to run Cal/Mag products until the end. This results in greener buds up until harvest but overall, a dirtier burn
Hope that clarifies things up,
Joel