Why doesn't Illustrator use the artboard dimensions when saving as pdf?

Hi all,

When I save as PDF with Illustrator CS, the dimensions of the resulting PDF do not match those of the artboard in Illustrator.
Instead, I think it uses the 'total' dimensions of all artwork in the AI file. So also artwork which is partly invisible (because of a clipping mask for example) is used to define the PDF dimensions.

So my question is:
Is there a way to save PDF files with the same dimensions as the artboard?

Thanks.