Just for contrast, my line is 520m, the attenuation 18.5dB and SNR 6dB. My errors are low and my connection stays up for weeks on end but I've never synced above 46M. Still 3M above the 'handback threshold' though. Just unlucky I guess.

It is just bad luck. It may have tried dropping to 5dB and found instability or maybe it sees a medium length line (at 18dB attenuation) with a fairly low sync for the attenuation and knows it will not be worth trying!. At 16.5dB over 450m I have sync speeds up at mid 70s but know of one specific interferer that takes 6-8 Mbps.

Yes, it is true that 3dB will sometimes drop to 2.x margins.

In my case when I was on EO Line 3dB was impossible to maintain stable sync. The internet connection wouldn't even last for half a day before dropping out. With 6dB it will also drop out within 2-4 days regardless of what ISP or router quality I buy.

In my case only a manual ISP cap of 9dB on ADSL was my connection stable and even then 9dB would drop to 3dB almost every day in the mornings due to some interference that would occur for years. After 10 minutes to 2 hours it recovers back to 9dB. But because the SNR is high enough the drop didn't go to 0dB.

I personally have observed in my router stats over the years with a 6dB or 3dB default SNR under ADSL formerly EO Line the SNR would never be stable and would very frequently drop to 1dB even witnessed 0.8dB and that gave me clues why my connection was dropping out. Like it would disconnect 5-6 times a day with a 3dB profile. No Openreach engineer or Sky engineer was able to fix this issue.

FTTC finally came and that is when miracles happened. Now the same 3dB SNR no longer poses problems and from my observation last 12 months since having FTTC. Noise margins are very stable. Extremely rarely have I observed even minor fluctuations and that's an indicator of a stable line thanks to shorter copper lengths I believe.

I think the longer the copper line to the cabinet the higher the risk there is to picking up noise and that can reduce speeds. Now my cabinet is around 320 meters and I have measured it with google maps. But my line attenuation shows as 17.90-18dB. Some have said that, my cabinet must be longer than that. Even if it is quite a bit longer than that my connection has almost always synced at 80Mbps.

There was only one occasion a few months ago following TalkTalk router firmware update that DLM dropped my speed to 79.6Mbps for 1-2 months and then it went back up to 80Mbps.