Although the Scott catalogue usually lists stamps in chronological order by date of issue, there are exceptions. When a country has issued a set of stamps over a period of time, those stamps within the set are kept together without regard to date of issue. This follows the normal collecting approach of keeping stamps in their natural sets.
In this case, Scott grouped the stamps for number continuity, but numbered the stamps in value order, not chronologically. Most catalogs do this to some degree.