Many/most cops(specially in urban areas) are not gun guys. They know little more than what is required to pass qualification with the issued firearm. Same with many military personel although they may have experience with more exotic applications.
I have military experience(precision rifle applications) but little of the rattattattat. Some military types with much more shootem up trigger time still come to me for firearms advice simply because they only know about what their unit is issued.
Cops are notoriously poor advisors pertaining to legal issues.