You're talking about the initial launch-hover, right?
I think there is some fuzzy math going into the launch and that causes the craft to make adjustments a few secs after the launch. Since the altitude is calculated by air pressure, and since the accelerating props blow air into the ground around the phantom, the air pressure builds significantly just before it lifts. Ordinarily that would mean the aircraft is dropping in altitude, but it hasn't launched yet. So the algorithm allows for this and tries to correct for that adjustment. Then after you launch, the software in the next few secs readjusts to what it believes is correct. So this will cause a little bouncing the first few secs but it should level out quickly...