Hi Hd,
Strictly as an amateur, my guess would be that BT have forgotten about the laws of physics. If you add a higher capacity fibre circuit to the existing infrastructure, it is likely that the traffic load will increase substantially.
When that happens there has to be some logic which restricts the throughput so at least the load stays within nearly manageable proportions. Someone else here might like to talk about the non-deterministic nature of broadband circuits. Using the Ethernet model (if that's still valid) it works quite well up to perhaps 60% or 70% but thereafter the transmission retrys rapidly swamp the network.
Kind regards,
Walter