I believe there's an excess of cases with RD/ND Jan-Apr 2002, because of the 245(i) "window" back in 2001. I judge of my own experience, since my case falls into this category. And while the majority of those case were transferred to local, there probably is a lot of them pending. I also believe these cases are not straight forward - if you know what 245(i) was, you'd understand. Once they were all done, the processing dates will jump probably to Aug-Sep, just in couple of months. This apperently doesn't affect approval of later cases.
Remember, this is only my personal interpretation.
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I-140 AD 12/23/2001
I-485 RD 01/24/2002
FP1 03/23/2002
FP2 11/01/2003
3 EAD (don't have the dates) approved
No RFE
LUD 08/16/2004
Online message regarding inquiry
No messages changes regarding FP2
No notification that case was transferred to local office