Found on an attoney's Web page
Posted: Fri Apr 16, 2004 9:01 am Post subject:
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I had applied for I-140 and I-485 simultaneously on March 26, 2003. The current processing date is May 2003. What could be the reason that my I-140 is not approved yet?
Attorney Murthy : Generally, the USCIS policy is that the date they mention is simply the date that the files are assigned to the USCIS examiner. It is not the date that the cases have all been approved yet for that date. So, the rule is that one is supposed to wait about 4 months from the date mentioned as the date having been processed since the examiner may take that long to get to a particular file.
(from murthy.com)