the last 2 numbers are usable screen space, the screen resolution minus 22 pixels height which is for the menu bar at the top of the screen (this space is unusable)

I definately wasn't clear in my original post :)

AFAICS, this really shouldn't have anything to do with cascading - like many others I am a devout user of tabs, and command click to open new ones. Because of this, I never need to have more than one window open, and yet quite often find that when I launch safari the window opens in a different position to where it was when i quit (i.e. off to the right.) Using the defaults command in the original post in fact has no impact on cascading, and new windows will still open slightly offset from previous ones.

I read an anecdote somewhere (quite likely here?) about apple engineers demonstrating an early build of safari and tearing their hair out over this 'feature,' which I think is why I was so convinced it was a bug.

Still, six and half a dozen I guess...

And regarding the numbers 685 and 746 in the original command, I believe (as someone already pointed out) that the latter is usable screen height, and the former is the intended default height of the safari window upon launch. It's shorter because I keep the dock visible at the bottom of the screen.