They've bypassed a few smaller exchanges with fibre. My own exchange - Brackley - isn't all that small (several thousand premises) but apparently we get our fibre service from Banbury up the road rather than our own exchange. I've no idea where our voice service links to but wouldn't have thought it was Banbury. I vaguely recall a fault a few years ago that affected us and Buckingham so assumed that we went via Buckingham to Milton Keynes.

I reckon BT just pick whatever seems like a good idea at the time