There are a number of reasons why the aircraft would return home early. One, the battery isn't properly calibrated and the aircraft didn't think it had power to go any further. This is independent of the number that is reported to the user. Read DJI's recommendations on how to calibrate your batterys. Reason #2 for why it came home early is that you maybe inadvertently selected a flight duration value in the app. Set that value to "Battery Limited" to go until the aircraft says it needs to come home.
Anytime you are using Terrain Awareness the RTH height is set to the highest area that experienced on the terrain height profile plus 40 meters for safety. When the aircraft is in the RTH state Map Pilot Pro is not telling it where to go anymore and the timing of that event can't be controlled so we have to add that buffer in there for safety.
