Unexpected Colour differences (MacBook Pro, OSX 10.11.6)
While I don't know the exact details, the Digital Color Meter that comes with macOS is affected by the color profile settings in System Preferences > Displays > Color. Changing the display's color profile from Color LCD to P3 to Adobe RGB to whatever causes a solid color to read differently under the same Digital Color Meter profile selection.

A photo editor with a dropper tool, specifically Photoshop, always selects the same color values, regardless of the display's color profile.