Why does the DJI Phantom 4 have a 3 mile transmission range if it works at 2.4 GHz ? Does it just output a lot of power. I thought the FCC limit the power to 1 Watt

I can't even get mine to go a half mile, I don't know where they get 3 miles from.

Range has very little to do with the frequency of transmission. Mostly it is about having exclusive use of the transmission channel and clear line-of-sight between the RC unit and the aircraft.

My best distance is 3 1/2 miles with LOS. Had to turn back otherwise would have run out of battery. Your distance will drop to under a mile or even less with obstructions. Years ago I knew a ham operator who talked to the space station with a 5 watt handheld.

I've been right at 3 miles with LOS and had excellent video excetra,,, , I've seen a lot of complaints about the new version and I haven't experienced any of this maybe because I use an iPad mini for one, and I have a lot of clear area to fly in .