Looks like USCIS issues a weekly update to calculate how many applications were received before and in early week and project when these applications are entered into system.
For example, On June 28, All I-485 were received before that day will be input into system by 7/20/2007. During week of June 28 to July 6, More I-485 applications were sent in, USCIS found that it is too many applications and it will take them until 8/1/2007 to complete the data entry. On July 13, They really found out there is no that many applications as estimated, They then adjusted the date back to 7/18/2007. Since people still send in the applications before and after the revised bulletin, On July 20, They issue another update that data entry will be completed by 8/1/2007 again.
Since more applications will be submitted, I guess that next update of I-485 receive date will be sometime middle of Sept or even later. These update provide people information if you know which date you ted your application, You probably will receive receipt after the application is entered into system soon.
It is just an analysis and hopefully it makes sense.
See USCIS ISSUES SERVICE CENTER RECEIPTING UPDATE for NSC from last few weeks.
07/20
I-131 EB 8/1/2007
I-140 EB 8/1/2007
I-485 8/1/2007
N-400 Military 11/22/2007
I-360 8/1/2007
I-765 8/1/2007
I-765 (Port of Entry) 11/22/2007
07/13
I-131 7/18/2007
I-360 7/18/2007
I-485 7/18/2007
I-140 7/18/2007
N-400 7/18/2007
07/06
I-131 EB 8/1/2007
I-140 EB 8/1/2007
I-485 8/1/2007
N-400 8/1/2007
06/28
I-131 EB 7/20/2007
I-360 EB 7/20/2007
I-485 7/20/2007