The SNR margin is the level of noise which is "acceptable" on the line.
On any given line the SNR comes down as the speed goes up with everything the same.
It is not that higher speeds are less tolerant of noise - it is that if the SNR is lower the line is more susceptible to noise.
If the target SNR is 6dB then this will determine the speed that can be achieved. If the actual noise level (not the ratio) the attainable rate will be higher. If the noise level (not the SNR) is low enough the line can run at the maximum speed (8Mb/s for ADSL, 24Mb/s for ADSL2+ if the provider will allow it) and the SNR will be more than 6dB (which is good) but a higher noise level will cause the SNR to cause the line to sync at less than the maximum.
It is possible to run with an SNR of less than 6dB on a stable line, particularly a short line which gives a higher line speed.
All down to absolute noise level which then determines the SNR for a particular speed (faster speed gives lower SNR as the bin loading on the line needs to be higher and more noise tends to be present on the higher frequencies used for the higher speeds.
Ex <n>ildram , been to SKY MAX - 15,225 Download
BE Unlimited - 21,000 Download 1,200 Upload ON THE LINE THAT SKY COULD ONLY PROVIDE 15,255 DOWN AND 800 UP ON!!!,
Moved house, now BE Unlimited 6,500 Down, 1Mb/s up - gutted!
FTTC Cab installation commenced 12th April - expect full 80 / 20 - bye bye BE, hello BT Infinity soon!