RD 8/1/01, ND 9/10/01 still waiting
Looks like the priority date (PD) also comes into play. Lots of August RD approvals have PD in the early 2000 range, although a few approvals with 2001 PD have also been seen.
The recent batch of approvals suggest that the INS is handling cases with PD in late 2000.
Of course, there may be a lot more dates in the equation, but I think largely this is the case.
Just my two cents worth.