Khalaf ibn Hisham al-Bazzar occupies a position nobody else in the canon holds. He appears twice. As a transmitter he is one of the two men who carry Hamza's reading, so his name stands under one of the seven. As a reader he stands tenth in his own right, with his own two transmitters, Ishaq and Idris, beneath him. The epithet al-Ashir simply means the tenth, and it is a label the discipline gave him rather than one he used. He worked at Baghdad, died in 229 AH, and belonged to the world of the Abbasid capital rather than to any of the older centres of recitation, which is itself part of his significance: by his lifetime the study of the readings had moved to the imperial city, where the material from Kufa, Basra, Medina, Mecca and Damascus could all be compared in one place.
Khalaf's double appearance is a small structural oddity in an otherwise regular scheme, and it is the kind of thing a reference work is tempted to tidy. It should not be tidied. It records accurately how the canon was built: out of the people who were actually available to teach, in the relationships they actually stood in to one another. The practical consequence is that a citation naming Khalaf is ambiguous unless it says which Khalaf is meant. Khalaf an Hamza is one of the twenty transmissions and belongs under the sixth reading. The reading of Khalaf al-Ashir is the tenth entry in the canon and has Ishaq and Idris beneath it. The same man, two positions, and a reader who conflates them will attribute a set of recitational choices to the wrong reading.