The dependent sign for short i attaches not to a letter but to a whole cluster. Where three consonants are joined into one conjunct, the vowel mark stands once, against the joined shape, because what it belongs to is the syllable and not any single character inside it. That is the point at which a reader trained on an alphabet has to change habits: the unit Siddham writes is the syllable, and a conjunct of three consonants carrying one vowel mark is one written thing rather than four.
Short i is among the most frequent vowels in the language, carried by a large number of grammatical endings as well as by the stems that take them, so the mark appears on almost every line of any Siddham text. Frequency of that kind has consequences for a hand. The signs a scribe writes many thousands of times are the ones that drift fastest, contract soonest and vary most between one copying tradition and another, which makes them correspondingly the most useful to a palaeographer trying to place an undated page.
The letter also stands behind the vowel e. The old analysis treats e as what results when a and i combine, so the strengthening grades the grammarians describe run upward from this vowel, and a reader who knows the rule can often see the short i inside a form that no longer writes it.