Some revisions are going to be necessary because of changing nursing practices. For example, there are several common drugs from a few years ago that are no longer on the market. In addition, the scope of practice for nursing is always changing, so they are always (at least every 3 yrs) looking at the exam to make sure it's still testing on the information that a new nurse would need to know for safe practice.
Go to
www.ncsbn.org and click on the '2006 Candidate Bulletin' for some more detailed explanation of how the NCLEX test plan/revision process works.