The flavor values are a bit screwy at the moment, so I don't find this too surprising on mid-low difficulties. Basically, low level buildings are less likely to be built if the civ has the technology to build high level ones. Most likely, the civ got the the writing tech so late all it had to build were high level buildings.
Edit: This is most likely to happen with civs having UBs, btw, since they will nearly always build those buildings first before others.