Djervi belongs to a small group of Glagolitic letters that fit the Slavonic of the surviving canon awkwardly, and that group is one of the arguments in the long discussion about where the alphabet was designed to be used. The mission Constantine and Methodius led went to Moravia, in the west of the Slavic world, at the invitation of its ruler and with the backing of the Byzantine emperor. The manuscripts that survive were copied in the south, in Bulgaria and Macedonia and later on the Adriatic coast, after the Moravian mission had ended and the disciples had been driven out.
So the alphabet was made for one Slavic region and is preserved almost entirely from another, and a letter that suits the first better than the second is exactly what that history would predict. The oldest surviving Glagolitic text, a small liturgical book, is usually said to carry western Slavic features for the same reason.
This is a matter of weighing evidence rather than of a settled result. The features can be read as survivals from the Moravian original, or as the work of later scribes in contact with the west. The atlas records the letter, records that its distribution is uneven, and does not present either reading as the answer.