Why is WiFi on 100% of the time, even though Extended Standby Mode is enabled?

My Sony Xperia S has an Extended Standby Mode which states that WiFi and mobile data are disabled after a time when the screen is off.

However, sometimes this stops working and the battery starts falling quickly, and BetterBatteryStats shows that WiFi was on 100% of the time for the last 22 hours. Usually after I reboot the phone it starts behaving normally.

This is the explanation text for the Extended Standby Mode:

And I know for sure that I don't have any downloads or uploads running.

What is happening here? How do I make sure that Extended Standby Mode really works?