A very high probability the loss is due to cross-talk .... ie: when other people off the same Cab opt to purchase FTTC broadband, your circuit will take a hit.
There's a technology called vectoring that can counter this to a degree .... basically it means a new card has to be fitted in the FTTC cab. But, only BDUK cabs were being retro-fitted with these cards ... not sure of the current position regarding commercial cabs ??
At least you've got the benefit of a sub 6dB SNR, otherwise your circuit would be even slower on 6dB+.