I do not understand why sometime PD has retrogression?
Is that because the PD is only valid for one month and visa is not assigned on FIFO basis?
In other words, if USCIS knows that some cases can not be processed in the next month, they are going to move the PD forward to ensure no visa is wasted? However, in the next month, when some of the old PD cases can be assinged visa (i.e., cleared NC), they reverse the PD date?