Weird curve...
So is this an LED backlit unit? If so, that's the issue.
Also try using Native White Point and Native Gamma, that should reduce banding if that's the only issue. Usually the White Point is pretty ugly on LED displays using these Colorimeters expecting a CCFL, sRGB sized gamut to measure.
You really don't want to be using a 1.8 TRC for the target calibration!