FTTN has the same sorts of limitations as ADSL, so poor lines, the exact distance (even as little as an extra 100m line length with FTTN can make a huge difference... it drops way quicker than ADSL does), all those sorts of things... so two lines with the same distance could still have different speeds because the node is more loaded on one, or because the copper quality is lower on the other.
If I am reading the attenuation right, 70Mbps is about right for the max line speed.