How does USCIS decide the "current processing date" value

[registrationid]

USCIS publishes the 'current processing dates' every 2 weeks. However we clearly come across cases in each of those categories that have receipt dates much prior to the priority date.
For e.g.:
Report 1: Week 1
USCIS publishes priority date for category A: 1/1/2004
Report 2: Week 3
USCIS publishes priority date for category A: 2/1/2004

On week 4 one can still find cases having receipt date of 12/1/2003

So how can USCIS publish a report on WEEK3 stating that they are processing cases applied prior to 2/1/2004 and yet have 12/1/2003 pending. Isnt it that only after approving all cases till 1/1/2004, USCIS is supposed to move ahead so that on Week 3 they would be working only on cases between 1/1/2004 and 2/1/2004?

 

[bharad1]

I read in one of the posts that CIS moves dates forward when less than 10% of cases filed before the date remain pending.