This is an odd example, but nationality law is full of odd cases.
All US Citizens are considered US Nationals, but it is possible to be a US National without being a US Citizen. This situation applies to some people from American Samoa (and Swains Island, with its population of 17 coconut harvesters), residents of the Northern Mariana Islands who have elected to be non-citizen nationals, and children of existing non-citizen nationals who pass on their nationality (subject to special requirements).
Very few people fall into this category (especially as some American Samoans will still obtain citizenship at birth through Jus sanguinis, and others will naturalize), and the State Department didn't want to spend money printing up special paperwork for them, so people who fall into this atypical classification are issued US Passport books that bear the annotation: "THE BEARER IS A UNITED STATES NATIONAL AND NOT A UNITED STATES CITIZEN."
As the Department has received few requests, there is no justification for the creation of a non-citizen national certificate. Designing a separate document that includes anti-fraud mechanisms was seen as an inefficient expenditure of resources. Therefore, the Department determined that those who would be eligible to apply for such a certificate may instead apply for a United States passport that would delineate and certify their status as a national but not a citizen of the United States.
This status occasionally causes problems, as non-citizen nationals may not be able to hold government jobs, vote, or run for office, at least not without obtaining citizenship, which requires the fee, test, background check, etc...