Why is the PSD landing page security zone width 950ppt and workspace 960ppt?

Question

Design

Why is the PSD landing page security zone width 950ppt and workspace 960ppt?
If you can elaborate...

Answer 1

Actually 940, and working 960!
In order to understand the root of the problem, one does not need to be a Yurodyev psychic. It is enough to think a little beyond the Photoshop window. The first thing that comes to mind is the 960.gs framework, Foundation, and so on.
As if after you draw it, the mythical character will type it all up. I understand, strange, unusual, but still. If you work with the 960 grid in advance, the layout designer will be able to quickly lay out the template using part of the ready-made code to create a grid, to work with headings, buttons, and so on.
It's pointless to describe everything in more detail, just try to make your own masterpiece using Foundation. This is a useful skill for all designers as lately I have the feeling that people are coming into this profession who don't even know there is such a css file.

Answer 2

Because some designer wanted it that way, having calculated the approximate width of the visible area on a certain average monitor.