As stated on this Wikipedia page:
Netflix's original productions also include continuations of cancelled series from other networks, as well as licensing or co-producing content from international broadcasters for exclusive broadcast in other territories, which is also branded in those regions as Netflix original content.
From another Wikipedia page:
These products, even though Netflix lists them as Netflix Originals, are programs that have been aired in different countries, and Netflix has bought exclusive distribution rights to stream them in other various countries. They may be available on Netflix in their home territory and other markets where Netflix does not have the first run license, without the Netflix Original label, some time after their first-run airing on their original broadcaster.
Netflix uses the term “original” to delineate between movies and series that are exclusive to its platform, and those that are aggregated from other studios after first being made available elsewhere. Original can refer to a few things: content such as Stranger Things that are self produced, programming like Narcos that are licensed exclusively from other studios and branded as Netflix originals, or licensed content such as You and Riverdale that may air on TV in some markets but stream first on Netflix in other parts of the world.