CRT (Cathode Ray Tubes) displays have a non-linear response to input voltages. This non-linearity is approximated by a power function called gamma, with a typical value around 2.2.
This non-linear response of CRTs is the reason why #6 was chosen for the 50% color component instead of #7 , to compensate for it.
In the same way, on a real CPC, the 50% signal has been measured to be around 42%.