AO, I suggest that you read up on geolocation - wikipedia will give you some ideas about what secondary information sources are used.
BT is not "giving out" any additional information beyond the IP address, it's probably just that the secondary information is more accurate.
It's also possible that BT have changed your IP address range recently to one that has previously been located closer to where you live.
When they can pinpoint your street and house number is the time to worry.
SD