Apple says it has made the change to ensure that features keep working
On your iPhone, switching off Bluetooth and WiFi doesn't actually switch it off.
The buttons found in the control centre – which has received a major redesign for iOS 11 – actually do something else, despite appearing to switch off the wireless technologies. Instead, when you press the buttons – both for Bluetooth and WiFi – they'll just disconnect from all the things they're currently attached to.
So switching off Bluetooth, for instance, will kick your headphones off from being attached to your phone. But your phone will still be sending out and receiving Bluetooth signals.
And while the phone won't try to connect to any Bluetooth accessories or WiFi networks for a short while, eventually they'll start back up again. Both will switch themselves on as normal if you move to a new location, the phone is restarted, or it gets around to 5am local time.
And it's all on purpose, according to Apple's documentation.
Apple says that it has made the decision so that various features continue to work. People might turn off Bluetooth because they want to unpair some headphones, for instance, but still expect that they're Apple Pencil will be able to connect to their iPad.
What's more, some important Apple features – like AirDrop for sending files, AirPlay for music streaming and the Continuity tools that let people move easily between different Apple products – rely on both. So those will continue to work even once the toggle has been switched off.
But some security experts have warned that having connections active with no easy way of turning them off could leave phones open to attack. Potential problems in Bluetooth that allow people to take control of phones can't now be easily avoided – and people might leave themselves exposed to those security problems while believing that they've actually shut their connections off entirely.
The connections can both be entirely disabled through the Settings app. If you do that, they'll turn off entirely and won't turn back on – though Apple says you shouldn't do that if you want "the best experience on your iOS device".