Hi Buzz, that's because the PC HDMI out is remapping the 0-1023 (0-255) range to 64-940 (16-235), as it expected, and as is correct for calibration!
If this wasn't done, the lower blacks from your PC would be clipped on most display.
This is true when using PC for desktop and stills presentation, NOT video footage.
When you use a software DVD player or similar on the PC the DVD window is treated differently by the HDMI signal chain - the same as with normal DVD players.
This is the function of EDID - Extended Display Identification Data - within the HDMI connection.
If you see problems with the levels it is likely the PC or the display is not providing or understanding the EDID correctly.
Placing a Lumagen in the path, and setting the input/output manually - PC or Video - can overcome such problems.
Mob Boss at Light Illusion