As I understand it that's the figures at which Openreach will let the ISP out of their contract, and refund any charges. I'm not 100% sure on when they use the clean or impacted figures. Looks like Talktalk are quoting you the impacted figure.
I recall reading something along the lines of, if you have a managed install (with an engineer testing the line) then they use the clean figures. If it's an unmanaged install (self install, no engineer) then they use the impacted figures. I can't recall if that was for the downstream handback threshold or just for an ISP quoting estimates. Don't quote me on that I might have read it on the back of my Coco Pops box.