Why did 405 look so good?

[Murphyv310]

Surely the write speed to the tube face was faster with 625 so there was less time for the phosphor to take the electron beam current and light up. The difference in write speed is close to 50%.
Try it with a CRT scope and you will see an extreme example as you change the time base speed. You need to turn the brightness up.

A fair point but 819 lines would have been grim by your reckoning with an even faster scan speed, the closer the line spacing the higher the subjective brightness, as you are scanning more phosphors.
Try increasing the height, the increase in line spacing reduces perceived brightness, reducing height give the opposite effect.