That seems to be common - and my understanding is that if the line is "idle" the power is "reduced" possibly by not using the higher frequencies and using just the lower ones to keep the line alive. The line data is continuously fed from DSLAM to end user modem so it will have a low power figure in the memory. By asking for line stats during a long, full speed download will give the 19.8 dBm, figure we are used to.
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