The difference, is that much smaller, when talking about the distance between, the cabinet, and the distance to your exchange.
FTTC delivers full speed up to several hundred meters from the cab , i believe.
When you get further than that, you drop speed yes, but nowhere near as badly if it were copper all the way from the exchange.
Not sure if anyone has posted a graph yet, of distance from cab to speed band. there used to be one for ADSL copper.
BT probably ran the numbers through a formula, they know the approx distance from each home to each exchange. those houses at the extreme from the exchange, are still going to be at the extreme, but just from the cab.
At any rate, its only the BT uplift multiplier, it doesn't really mean jack. All you really want to know is, 'how far am i from the cab'. and what speed will i get.