I've noticed this issue, too. When I go to build a new town (I haven't noticed common factors, like number of towns built before, amount of food, or location, or anything) and start adding improvements, the screen will show available tiles surrounding the town. However, it will only let me place 4 structures. When demolished, it still shows all tiles available around the town, but I'll open the town display and it will state that only 4 tiles are available.