Inefficient motor?

@sportykev, Four motors, all stop spinning at different times. Unless it's causing stability issues, I wouldn't give it a second thought. I have five quads and a hex. None of the motors stop spinning at the same time.

My X-1 was exhibiting a very similar pattern to yours, but it was affecting the performance. #3 motor (left rear) would stop immediately upon throttle off. In flight, the quad had a tendency to drift to the rear and left, so it was obviously an issue. In my case it turned out the motor had some carpet fibers wrapped around the shaft. The fibers were causing drag when they obstructed against the motor housing. Cleaned out the fibers, and the problem was resolved.

But... The X-1 uses sealed, brushed inrunner motors. This isn't that likely to be the case on your outrunner Phantom. However, it is possible to get debris like grass, dirt, etc. into the channel between the fuselage and the motor housing. Since it's an outrunner, this would have the same affect I experienced with the carpet fibers on the X-1. Have a look under bright light and see if maybe you've got some grass clipping or mud or something wedged in there and gently clean it out (or remove the fuselage top and do a more thorough cleaning).

I wouldn't worry about it though unless it's affecting stability.