Why did 405 look so good?

[Synchrodyne]

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.

Let’s say that the respective 405 and 625 line scans take about the same time for a complete frame, assume a constant video level somewhere towards peak white, and that the energy available from the electron beam is thus more-or-less fixed on a per time basis.  Then the energy delivered to the screen phosphor per frame would I think be more-or-less the same for both cases.  Would there then be a difference in perceived screen brightness?  I suspect not much if any.  Or is there a flaw in this argument?

 
 
Cheers,
 
Steve