your D1 looks unusual, trying to figure out where your adsl power cutback is, it weakens up to about tone 500 then jumps up again suggesting a possible power cutback at upper limit of adsl2+ range, but I am guessing you quite far from the exchange so that would be unusual,
Does look rather strange, the longer lines are meant to have less PCB.
...
It just wouldnt make sense to me to PCB a long line up to tone 500.
As far as I am aware, the PSD mask chosen for the cabinet is based only on the distance between cabinet and exchange. The remaining length of the line to the customer doesn't matter - so all lines on the cabinet get the same PSD mask.
If the cabinet is fairly close to the exchange (as this one seems to be) it will get a mask that goes up to tone 512 (because ADSL2+ tones will still be usable that close), but will cut the power by a lesser amount (because the ADSL2+ signals won't have attenuated by much). This would be the mask for CAL=10 in the ANFP.
If the cabinet is further from the exchange, then the higher ADSL2+ frequencies won't get through that far, but those that do will be attenuated more. To cope with this, the PSD mask alters by cutting power in a more extreme way, but then allows VDSL2 to use full power again at a lesser frequency.
The rules for setting the mask come from NICC's ANFP; I don't recall any special exclusions that allow a mask to be "reduced" if the exchange in question doesn't (currently) support ADSL2+. It ought to be feasible, but it obviously adds to the admin work if/when someone brings ADSL2+ to the exchange; and you'd have to explain to everyone why their current FTTC service lost 5Mbps
I wonder if in view of the relatively small amount of subscriber lines it wasnt economically viable to install 21cn equipment and backhaul upgrade..
In the long game, with copper being retired eventually, you'd imagine that the small exchanges (and their buildings) are no longer needed. Fewer exchange buildings are needed to host the head-end equipment, as the fibre signal can go a lot further than copper (20-30km vs 6km?)
I thought quite a lot of exchanges were affected by this kind of thinking; certainly my original FTTC exchange (Brookwood, Surrey) got FTTC quite a while before it became 21CN, so it must have been a child to somewhere - Woking or Guildford presumably.