A WinDIB fullscreen question

I made some tests and I noticed that,
if I have a program that runs in 640x480x256 fullscreen,
in WinDIB mode the program switches to fullscreen mode
only when the screen is set to 256 colors too.

If I run my program with screen set to 1024x768x64K,
it is able only to run in a window.

Someone can tell me why? Is it a limitation of
fullscreen in WinDIB mode, or is it a bug?

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