iMac display RGB values

Why does the iMac display show non-zero values for blue and green pixels for a .dng image whose blue and green color planes are zero? I used the digital color meter to show the non-zero values for the blue and green pixels and I confirmed that the .dng file has zero values in the blue and green color planes using python scripts that open the .dng file into a numpy array and then inspecting the array values.

Thank you in advance for your help on this matter.