I really wouldn’t worry about this.
The manual states:
Values under 3 are generally considered excellent for display calibration. Values above 3 will be highlighted in yellow or red and may indicate a problem with the display’s calibration.
The graph shows the displays actual measured grayscale color tracking using measurements taken while generating an ICC/ColorSync monitor profile of the display.
The horizontal axis shows the Delta E values for each measured point on the grayscale. The vertical axis shows the luminance for each of the measured points on the grayscale.
Now this note might be telling:
Note: Since the Delta E values are calculated based on the currently selected Target, if a Target using Intensity: Specific Level is being used, and the display was not able to actually achieve the Target’s specified Intensity value, then this will result in large Delta E values.
So, what's the specific level being asked for?