My understanding is that they are not allowed any clenbuterol in their system, presumably not a naturally occuring chemical/hormone (unlike testosterone etc).
So in the alcohol example, it is like being told that your blood alcohol limit is 0.00, but being tested by a method that can say that you have 0.0001 (ratios not aligned to the 400 times), but the requirement being that they only need to be able to test to two decimal points.
But, to be accredited as a UCI testing lab, they need to be able to detect a level of x, while Contador was detected with a level 400 times (??1/400th??, I dunno) less than this level. So a lower quality lab could still have been accredited by UCI and not have detected it and he would have been fine.