As Kevin has said the noise in each exposure is different and can therefore be cancelled out.
Pixinsight has a great little example of the process at the following page, ignore the math and blurb if you wish but scroll down to Figure 2 and check the links to the right of the example image.
This shows the effect nicely.
http://pixinsight.com/doc/tools/Imag...tegration.html
Edit: it might take some time for the images to load, their site seems to be very slow.