It will have sync'ed at 3dB at a low noise time. Now the noise has increased your margin has fallen.
Attainable is (in essence) continuously calculated by comparing the sync speed with the Noise Margin and working out what the speed would be if you had re-sync'ed again (which would be at 3dB) at the instant you read the stats. If you leave the modem refreshing that page you will see SNRM and attainable increasing or decreasing together.
If it doesn't auto-refresh every few seconds, refresh it yourself
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