Brightness is a non-local effect. i.e. if you change one pixel, the rendering of the adjacent pixels can change.
Also, the rendering is different depending on how the signal made it into the CRT; in particular composite video does not preserve certain things about the image, and in some cases developers took advantage of this/
Most famously the genesis had a poor composite encoder, which was taken advantage of by many games; Sonic 3 in particular looks bad even on a CRT with a real Genesis if you use RGB or component output.
Even color is a non-local effect. Turning two colored pixels on side-by-side on an Apple II makes them white. I can give you an unbounded amount of detail why :-)
Also, the rendering is different depending on how the signal made it into the CRT; in particular composite video does not preserve certain things about the image, and in some cases developers took advantage of this/
Most famously the genesis had a poor composite encoder, which was taken advantage of by many games; Sonic 3 in particular looks bad even on a CRT with a real Genesis if you use RGB or component output.