An alphabet is two things at once, and they can disagree. It is a set of signs that appear in writing, and it is a list that a community recites, teaches and counts. Usually the two match. In Mandaic they do not, and kad is where they come apart.
The traditional count is twenty-four and it does not include kad. The modern character standard encodes twenty-five and it does. Neither is wrong. A recitation is about what the tradition holds its alphabet to be; an encoding is about what a manuscript might contain and what an edition might need to reproduce. A sign that appears in real writing has to be encodable whether or not it is a letter.
The same tension produces much larger discrepancies in Syriac, where a character inventory runs to far more entries than twenty-two because it carries ligatures, positional forms, Garshuni signs and letters borrowed by other languages entirely.
The practical rule for reading any alphabet entry in this atlas is therefore to ask which question a count is answering. Twenty-two is the inherited sequence and the number to use for comparison. Twenty-four is the abagada as the tradition keeps it. Twenty-five is the encoded range. This atlas holds twenty-five rows and records on each one where it stands in the other two reckonings, which is the only arrangement that does not quietly overrule somebody.