[bemmu]

In the Atari TOS screenshots, other icons such as arrows are black on white background.

If the icons in upper left and right are also like that, then the upper left icon is actually four little triangles pointing inwards and not an X. The one on the right is four little triangles pointing outwards.

(Or it could be an X)

[empressplay]

Atari ST users seemed to see it as an X -- so whether or not it was intentional on the part of the designer, it's not that hard to see how one of these users could have then translated that misconception to the Windows 95 GUI. It's all about perception...