I'm convinced you made a major error your color measurements. People's measurements on all of Apple's other P3 displays are dead on.
I think the problem is that the color gamut is NOT DCI-P3 but Display P3. DCI-P3 specifies a 2.6 gamma, but Apple uses a 2.2 gamma due to computer use. This accounts for your consistent error away from the white point.
The second potential error (probably less significant) is you used a colorimeter. Wide gamut displays, like Apple's, do not use conventional white LEDs, but use BG phosphor technology. The different spectra means that a different calibration matrix needs to be applied to the filters in the colorimeter. If you used a spectrophotometer, like the ColorMunki Photo, you can eliminate this issue.
Unfortunately, you've now started a rumor that isn't true.