It's uncommon, but content namespaces may sometimes include some JS/JSON/CSS pages, which obviously can't have categories.
One example of such pages is per-work CSS pages on Wikisource, introduced by T276067 (like these: https://it.wikisource.org/wiki/Speciale:PagineSenzaCategorie).
These pages should not appear on Special:UncategorizedPages, based on their content model.