This is a better link with a better explanation:
http://www.usmle.org/General_Information/announcements.aspx
"The change in scoring procedures will introduce a more stable relationship between score scales in the future. Those receiving 2-digit score results under the new system will note that, in most instances, the 2-digit score associated with a specific 3-digit score will be substantially lower than it was prior to this change.
The change in scoring procedures described above does not correct the challenges associated with using the 2-digit score scale for decisions other than those that use a score of 75 or higher to identify examinees who have passed the examination. It is highly recommended that the 3-digit score scale be used for comparisons among examinees, especially for examinees who have tested at different points in time."