The Meiri [Megilla 2b] says that people in a city which is not clear whether it is walled from the time of יהושע בן נון should read on the 14th because MOST cities are not walled and thus read on the 14th.
What about כל קבוע כמחצה על מחצה דמי? Every city is קבוע??!! So it is 50-50 and there is no רוב!!?
