first sorry for my english.

Why isn't it possible to make smooth gradient backgrounds with photoshop?

I know just one technique (add grain) to make it a little more smoothier but sometimes even this won't work.
So anyone can tell me why this is?

Some example pages:
http://www.designbombs.com/
the background on the left menu

http://mediatemple.net/
the gradient left and right dark corners.

thanks