Price per square foot is only as good as the quality of the sample size & selection. If several smaller homes on one level close, the price per square foot will go up. If larger, multilevel homes close the price will go down.
The only data you should use for decision making is the data drawn from a quality Broker preparing a Comparable Market Analysis from the most similar comparables. Zillow, Trulia, Eppraisal, Core Logic and others are all interesting, but not reliable for a specific home.