Not all statutes are codified, that is, not all laws passed (by Congress and signed by the President) appear in the numbered sections of the U.S. Code. Sometimes they appear in the code section’s note.
This blog post may not mean anything to you right now, but one day you’ll say “so that’s what Laura was talking about!”
I used to teach this to law students, using the Privacy Act of 1974 as an example, but a federal agency law librarian has written a useful and short memo on the subject so you don’t need me anymore (at least not to explain this!):
The Law Librarian’s Society of Washington, D.C., Inc. (LLSDC.org) has placed on its Legislative Source Book a PDF version of the presentation made by Rick McKinney, Federal Reserve Board Assistant Law Librarian, on “The Authority of Statutes Placed in Section Notes of the United States Code.”
This occurs in state codes as well. There are a number of perfectly good reasons why not all statutes are codified, but those statutes are still law.