I don't think if you instructed someone to make a UI with greyscale 3D bevels it would look as close to NEXTSTEP as Windows 95 was. There's specific things like the source of light being on the top left, etc, which make them nearly identical.
Then it could be straight from the top, but you only get one side lit and furthermore it's not very natural as it happens rarely.
Then it's only two choices, from the left or from the right. A coin flip? Actually no. Top left lightning has a long history for some good reasons https://en.wikipedia.org/wiki/Top-left_lighting so we could say it's the only professional and safe choice
No, you could have the top highlight be 2 pts deep and the sides be 1 pt and one shade darker, or something like that. There are many other ways to do it.
> and furthermore it's not very natural as it happens rarely
Every day from 10 am to noon (1/3rd of the day), and any time you have your back to the sun?
> we could say it's the only professional and safe choice
Mac OS X has top down lighting and it looks great. I think what you are trying to say is that you can't think of any other ways to design the buttons.
Anyway, top left lighting is not the only detail they copied. BeOS had top-left lighting and still managed not to look like an exact clone of NEXTSTEP.
http://toastytech.com/guis/ns33nfsmanager.png
(3.0 released Sept 1992, 3.3 Feb 1995)
Windows 95 released Aug 1995.