Why distinguish the reported “kadhdhāb” for Muḥammad b. Saʿīd from “not thabt”?

Narrator Criticism, Integrity and Precision: Standards of Scholarly Appraisal

• Kadhdhāb and non-thabt are synonymous grades of deficient precision
• The first is a severe accusation of lying, the second concerns deficient accuracy
• The first refers only to poor retention, the second necessarily proves deliberate lying
• Different critical terms can be treated identically without examining their evidentiary basis