The twenty-second place is a boundary, and it is the same boundary in four alphabets. Hebrew, Syriac, the Arabic abjad reckoning and Mandaic all inherit a sequence of twenty-two letters, and all four then do something different with what comes after.
Syriac added nothing. Its alphabet is twenty-two letters and has been throughout its documented history, and everything that looks like an addition is a ligature, a positional form, or a sign made for another language.
Hebrew added five final letters, counted apart from the twenty-two and given values in the hundreds.
Arabic added six letters for sounds the sequence had no place for, and appended them after the twenty-two in its ordinary alphabetical order while keeping the old order for its numerals.
Mandaic added two ligature letters, dushenna and kad, and then a borrowed Arabic one, and closed the traditional count by writing its first letter again.
Four scripts, one inheritance, four answers. At is where all four stop agreeing. Everything before it can be compared place by place with confidence, which is why the relations this atlas records between these letters run only through the first twenty-two, and everything after it belongs to the individual history of a single script.