Cyrillic shta writes the same cluster and, like the Glagolitic letter, carries no numeral value in either system. Neither alphabet could give it one from Greek, because Greek had no such sound, and Glagolitic had already stopped counting before reaching this part of the order.
The two letters make the same point about the design of the alphabets from opposite directions. Both scripts needed a sign here, and both put one in the same place, at the Slavic end of an order that follows Greek for as long as Greek is any use. That agreement of arrangement, in a stretch where Greek offered nothing to agree about, is one of the strongest pieces of evidence that the two alphabets belong to one tradition rather than to two.
The Cyrillic form is generally described as built from the letter for sh with a further element beneath, which makes it a compound rather than a borrowing, and the Glagolitic form is not obviously a compound of anything. Where the Cyrillic signs for the Slavic sounds came from is the same argument as where the Glagolitic signs came from, and it has the same three candidate answers: the other Slavonic script, a Semitic model, or invention. The atlas records the pairing and does not choose.