Crosstalk and the quality of the pair which a line uses I guess are the main factors. My current two lines both differ somewhat.
For example, one line has an upload sync rate of around 16 Mbps while the other is around 12 Mbps. One of my lines has not far from twice the amount of errored seconds compared to my other line.
Line 1 (DrayTek Vigor 2860 using DSL driver 5.7.6.13.1.7):
Downstream ES (past 24h): 413
Downstream SNRM: 4 dB~ (offset -2.0 dB)
Downstream Sync Rate: 67,654 Kbps
Upstream Sync Rate: 16,146 Kbps
Line 2 (ECI /r with LEDE using DSL driver 5.9.0.12.1.7):
Downstream ES (past 24h): 651
Downstream SNRM: 6 dB~ (no offset)
Downstream Sync Rate: 57,753 Kbps
Upstream Sync Rate: 12,881 Kbps
ECI DSLAM (ugh) on line card running version 0xd086 (208.134). No G.INP yet, sadly.
I've swapped modems around on the lines in the past just to double check one isn't performing better than the other and that's not changed much really. I could probably reduce the downstream SNRM target a little further if I wanted to get a bit more speed out of the first line. Obviously you aren't meant to do this, but it's not as if it's harming anyone. If I get a line fault develop then I'll revert it back to using no offset
.
At the previous house I lived in until early this year, both lines there significantly differed in performance.