Even though your observations seem meaningful and based on facts, I have to disagree with you...
The cases waiting for assignment are held in shelves sorted by month of RD. If that is the case, I cannot imagine how they would track cases by WAC # which is really the ND. I think what we are seeing is that May, June and July cases have been assigned simultaneously. A lot of June and July cases fall into the WAC-260+ range. So, its reasonable to expect that June and July cases being approved, fall into that range. I have always stuck to the notion of processing by month of RD because of how cases are held in the "work distribution area". To have to process cases by WAC/ND efficiently, they would have to sort their cases by range of WAC #. Maybe they do that, but all information at my disposal suggests that they queue cases by month of RD.
I know I can only conclusively say that processing is by month of RD, If INS processed only 1 month at a time. But, if there are no more cases ready to assign in a given month and they have officers with time on their hands, they have to assign subsequent monts\' cases. That is why we see approvals spread across a few months of RD at any given time. My 2 cents.