In old map preview it is mentioned that the EB map is based on the Barrington Atlas of the Greek and Roman world. However, I imagine that there is little data on cities outside the Greek and Romans sphere. In fact, some areas of the map would have few if any cities. Also, EB strives to give each city the local name, as opposed to the name the Romans/Greeks gave to it. This means they sometimes have to reconstruct a name. I also know several names are placeholders.

On a related note, isn't Palmyra (in Tadmor) a Roman name? IIRC the town was called Tadmor as well.