The U.S. Congress is not the only source of Federal documents. The Executive branch, headed by the President, consists of many departments, including the Treasury, War, Justice, Agriculture, and State departments (among many others). In the 1990's CIS published an index to the documents produced by these bureaus, agencies, and offices. They also republished the documents not found elsewhere in microfiche format. Today, CIS is part of ProQuest so now you can choose to use these materials online through ProQuest Congressional as well.
While all titles are available in PQC, not all titles are findable in IUCat. Those records that are in IUCat will be connected to the microfiche, not the digital copy in PQC, but you can search the title and date in PQC to find them there.