Some areas in the City of Richmond are within the 94806 ZIP Code, which the Postal Service lists as "San Pablo".
Postal Service delivery areas and ZIP codes are set up for mail delivery and do not necessarily follow actual municipal boundaries, so the city listed on a mailing address is not always the city in which the address is actually located. If you consider that city boundaries change over time, can contain a handful or millions of addresses, and that many addresses are not even located in incorporated cities, you get a sense of why the Postal Service wouldn't bother trying to follow those same boundaries.
Rest assured a house in Richmond is in Richmond even if its correct mailing address says "San Pablo". A house with a San Pablo mailing address may be in San Pablo, Richmond, or in an unincorporated area of Contra Costa County like Tara Hills. The Postal Service considers "Hilltop Mall" and "Richmond" as acceptable city names for 94806 (http://zip4.usps.com/zip4/), but finds "Tara Hills" unacceptable.