Monica,
Sorry if I confused you.
It is not that the I-485 adjudication takes a year (although it may take several months).
The Cut Off Date in the VB has to be later than your PD for 2 crucial parts of the process:
a) To be able to submit the I-485 and have it accepted by USCIS.
b) For the I-485 to be approved when it is adjudicated.
As we have seen before, dates can be Current to allow (a), but then retrogress. The final approval cannot happen until the dates are Current again. For July 2007 filers, this has been several years already.
The (rare) exception to this is if the visa number was allocated when the PD was Current, but the approval date is shortly after the dates have retrogressed.
So, yes, subsequent retrogression can delay the GC, if the Cut Off Dates retrogress to a date equal to or earlier than your PD.
Especially for new filers, if the dates are retrogressed shortly after the filing date, they may well find that they can submit their I-485, but it can't be approved until much later in the year, because EB2-I only has 2.8k visas initially in any FY. Only when sufficient spillover numbers become available, can the I-485 be approved.
Especially if DOS want to build a pipeline of future cases, this is a distinct possibility for many filers with later PDs.
I hope that hasn't confused you even more.
