I believe it's the result of the reasons for the 28 degree C policy which is a factor of:
1. Population has lower body fat than Westerners so they tend to feel cold more easily.
2. They can't afford or have access to enough energy (with the power plants being off line and having to import energy resources) to cool the country to levels seen elsewhere.
Therefore there has been acculturation which I believe is both biological and group psychological to having warmer temperatures in the summer.
In the winter as only Hokkaido is generally centrally heated, the opposite applies. Other than a common room where people sit together under an electric blanket, most of the house winds up being in the high 50s F and you can see your breath if you are not in the space and blanket heated common room.