It is also possible that this is the subfloor and the regular floor was never installed. This was a normal installation prior to the 1960's for floor installations. The original homeowner, as a way to cut down the cost, may have said that he would install the hardwood later.
One way to tell is by looking at the quality of the wood. If Henry is correct on the wood shrinkage, I would inspect the house much closer for other signs of poor workmanship and poor materials.