You're getting close, very close in fact
You are slightly mis-reading David's post, because of your remaining bit of confusion, and that mis-reading increases your confusion.
Have another read of my main Noise margin page
In short, you sort of explained it correctly a few minutes ago. Say there is a Signal to Noise Ratio of 40dB at the time you connect. That 40dB is the most important figure. Your sync-time margin is then used together with that to choose a speed.
If the real noise goes down, the SNR goes up and so does the reported SNRM at the same time. It therefore follows that if your SNRM has risen from 15dB to 17dB that the real noise has fallen and that the SNR has risen.
By doing a re-sync at such a time, because the real noise is less than the previous time, the sync speed will be higher, as that higher SNR is used with the 15dB figure to decide the speed.
The "gotcha" in that is if the noise margin is varying quite a lot at the time. You may catch a bad level of real noise instead of a good one. The higher SNRM on RouterStats needs to be looking stable. By that, I mean over several minutes with no real spikiness.
Does that help?
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