Well to explain this discrepancies that many of us have one must know the following:
1. That the data USCIS is providing is accurate
2. The definition of their data
So if we assume that the number of pending N 400 is not changing but the time to get naturalized is shortening (as it is really obvious, I remember not so long when people were waiting for 3 years and more) than the bottle neck seems to be in the local centers. That means that Service centers were always pretty fast and were handing over the case (after FBI clearance and N 400 check) to the local centers in a matter of month or two. This is hard to believe but it looks like it is the current case. That means that it was always the local center that was determining the speed of the process and looks like major improvements were made on the local center level. So if there are any problems it must be on a local level. Yet, the fact that Service centers are not decreasing the number of pending N 400 and in fact the number is increasing is very concerning.
So to answer your specific question, we first must know what the definition of current date means. If someone knows it let us know here also. I remember reading quite awhile ago that the current processing date means the date the latest application was lifted off the desk. That does not mean that other dates are not processed means that the newest application somebody was working on was the date they put on their web site. This definition probably holds true as it is impossible to explain how somebody was working last month on let say 07/2004 applications and this month on 05/2004 applications.
So in your case the regression can be explained with the fact that the center has seized activity completely as far as adjudicating arriving cases. Whoever was working on the older case that is the number (date) they will post on the web site. The fact that this is regressing on a month to month basis means that they have pretty much completely stop working on scheduling interviews.
