Unripe orange??? Are you asserting that ten unripe oranges picked at random will exhibit the same reflectivity?
I'm sure his subjects were chosen to exhibit the greatest possible variation.
Fred Picker was a businessman. His newsletters (I have and have read all of them) were intended to support his marketing efforts.
His newsletter was his blog (before such things existed). It was not a peer-reviewed journal, nor in any way vetted.
So the meters he tested were right more than 75% of the time. His mods only reduced the error by 50%.
You need to assess the information being presented, not just buy into the marketing hype.
Photography attempts to establish processes and standards that work in most situations involving light and objects
that are totally uncontrolled. As such assumptions and generalizations must of necessity be made, as in any physical science.
It's like the use of an 18% gray card. You can find innumerable scenes with reflectances that differ from that standard.
As to your last statement... I have not said one word about Zone VI meter mods.
In any technical discipline, you're expected to use tools and methods appropriate to the task.
When you choose to do otherwise, your results are yours and yours alone.
The Macbeth chart was never designed for use in the IR or UV ranges, as I stated previously.