You are right, if my memory is right,the law only says when the world is not overscribed, the extra number will be used for the overscribed countries. The law does not set a rule regarding how to distribute the extra number among the overscribed countries. DOS evidently distributes the extra numbers based on the number of applicants in each overscribed country.
I am disappointed at the visa bulltin too. I expect a greater movement than it is. However, the good thing is that movement of EB1 and EB2 is maintainted and EB3 passes 245(i) deadline. It is possible that DOS uses extra caution to avoid last year's embarassing situation (no number for EB3 for the entire fouth quarter) in handling EB3 to see the effect of 245(i).
There is another factor that may also affect the pace of PD movement. See the following breaking news from www.immigration-law.com
Our record reflects that the Backlog Elimination Centers are indeed picking up the speed of adjudication of the backlog labor certification applications. It is indeed a positive sign. This changing pace of processing of backlog cases is significant in that some of these cases are expected to give relief to the employers in the form of substitution of alien beneficiaries as well as resolving the issues of visa retrogression for their alien employees before the agencies enact and implement the elimination of the substitutions. Understandably, some employers have refiled and obtained new labor certifications under the PERM program without withdrawing the pending backlog cases. Employers have been receiving the large "white envelopes" in the mail in increased numbers from the BEC.