That's because real vacancy (as in, untenanted) only makes up a small percentage of the empty houses. The rest are owned by wealthy foreigners who reside there a few weeks in the summer and leave it to the care of a caretaker (who does not live there of course) for the rest of the year. Some of the obscenely wealthy may even have a live in concierge who lives in his own rooms keeping the rest of the house ready for its owners all year round.