The article below explains why some people need two shots. It seems the number of shots is controversial, that US policies don't jive with WHO policies -- doesn't exactly inspire confidence that they know what they are doing. Seems they can't tell for sure whether the one shot is enough, so in many cases recommend two, taken some time apart.
The more I learn the more suspicious I am that they have no clue how this affects Crohn's, for better or worse:
http://blogs.sciencemag.org/scienceinsider/2009/11/2-shots-for-kid.html