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You will only get consistent tunes values (-4, -4, -4) if you don't touch the camera at all. The second you move it a tiny bit or refocus, you now have a new tune value. How do you know if the single tune value you achieved on your only focus attempt at the dead center of the target is best value when it changes a little bit every time you refocus or point the camera at a different view of the SAME target.
I still say that a range of tune values should be taken and averaged.
I just re-checked my results and you are right, changing the light levels (all in the day though... just noon vs. afternoon) or slight movement in camera changes the results completely.
This is what I conclude: phase detect AF can never be as accurate as contrast based AF detection.