The USCIS works in mysterious ways. My oath was 5 months after my wifes, but so was my interview. You might try contacting the DO to see what is going on (by phone if you have a phone number, or an infopass).
I had a 3 month old Visa bill show up in my mail box the other day. The USPS is pretty reliable, but they screw up occasionally (and, the USCIS does it regularly).