waitingforgcard,
I think you are right about RD processing. Based on my understanding, if ABC's PD is June 2005 and RD June 2007, while XYZ's PD is December 2005 but RD is June 2006, then USCIS will process XYZ's case first, based on RD (considering that both PD's are current).
Unfortunately, other factors like name check and internal review of USCIS do delay the cases and as a result we do not see the a pattern based on RD.
Nowadays, USCIS starts processing cases as soon as they get them. Like initiate name check, FPs etc. So, having a RD 9 days later should not make much difference at all for processing your case. But if you feel strongly about it, you can provide USCIS delivery proof and they should be able to correct it.