Here muṣaḥḥaf and muḥarraf concern a chain or text whose general form remains, but one or more letters change so that it differs from the reliable narrator’s report.
• Muṣaḥḥaf: a word changes through its dots.
• Muḥarraf: the case of a change in a letter’s form.
• In the example, reading سِتًّا as شَيْئًا is taṣḥīf.
The focus here is a change in reading, rather than an addition or rearrangement.