Draytek Support (very helpful & prompt ) have advised after checking the data from the router that the Attainable figure is being calculated from the "standard" snr of 6db, but that as my connection is connecting at a higher snr (presumably because it has a 9 or 12db target snr for some reason) then that means the current speed will be lower.
Once the line starts connecting at 6db then the attainable speed will be attained :shock:
So it seems as if the attainable figure shows the theoretical speed that would be achieved should the line connect at a "standard" 6db snr :wink: