Attenuation is an estimate only, its normal for it to fluctuate between resyncs.
Yup, my line attenuation dropped from 5.4 to 4.3 after power cut next the next resync. it will rise again to 5.4 I used to it though I do not know why it fluctuate with a big difference!!
Fiind it strange that it flucturates a lot i always thought attenuation is how far you are from the exchange? Unless the exchange is constantly on the move.
But in our case as we both are have FTTC, the cabinet would be the moving thing.
The actual attenuation does vary seasonally and with temperatue but with a the same modem and firmware anything other than a small change usually means a change in the line. However different modems and firmwares do give varying values which are just due how the modem deals with the dsl signal. I also found the "x6" firmware gives a higher attenuation but for me a smaller incease in sync of about 0.3Mb/s.
Attenuation is resistance in the line so yeah, even a small change in temperature causes it to shift. Plus there will always be a margin of error in the detection system, its been suggested some modems utterly fail to detect it properly.