This behavior is seen when comparing the
GSS6100 or GSS4100
against any other simulator type e.g.
GSS6300, GSS6700, GSS6560, GSS8000, GSS7700, STR4760
The GSS6100 and GSS4100 have a lower quality OCXO and a wider loop bandwidth on the synthesizer resulting in a higher phase noise and a higher noise floor. At maximum power -115dBm both of these issues cause additional noise on the signal resulting in a lower C/No value (around 1dB) compared to any other simulator type. As the power level is decreased to the nominal level of -130dBm C/No values converge.
When an issue arises about a measured C/No (SNR) value it is important to check the calibration status of the unit and often times a re-calibration is required to give confidence in the simulators output level.