I don't think that is the reason at all. I think they just do it (sometimes) so that they can separate the processing and passport production operations. According to the insert that comes with the passport, passport cards will always be sent separately of anything else whereas passport books and supporting documents may or may not be separate. Therefore, you can get between one and three shipments depending on what you requested and how they handle it.They return the passport and naturalization certificate separately to preserve some measure of redundancy for you in case one of them gets lost, misdelivered, etc.
I don't think that is the reason at all. I think they just do it (sometimes) so that they can separate the processing and passport production operations. According to the insert that comes with the passport, passport cards will always be sent separately of anything else whereas passport books and supporting documents may or may not be separate. Therefore, you can get between one and three shipments depending on what you requested and how they handle it.
from which state did you apply for passport? looks like it took almost five weeks for you .... i am in the same boat still waiting ....Any update on your status, I received my PP and NC on the same day