At times I have noticed altitude readings being as much as 6 feet when defending to land after a good eratic FPV flight. normal?
At eye level it is easily to visually notice the altitude is being off.
Next question is to the accuracy and consistency of relative way points. I have been wanting to do my own very low altitude missions with hover on point times long enough top roughly plumb ground markings in relative points or same  points flown under different times or conditions..has anyone already tried this and if so the results?

As the Phantom warms up it is common for the barometer to be out by a small amount like yours.

May I know which drone do you refer?