I know, I never expected to get such high speeds and my router maintaining them too. Maybe its because I am on the edge of speed that I see such a seasonal variation, the very best is only possible in winter and I lose 2 db+ of SNR come the warmer weather. Of course the SNR increase gave me the ability to get the 7 mb speed. Cone April/May, if it's the same next year, SNR will drop and so will my speed.
The idea of atmospheric moisture explaining it, the cable is underground and we've had a wet and/or humid summer. A wet winter in theory would cause speed to drop closer to summer speeds.
What amazes me is that this is only plain old ADSL1 and I'm not that far from the max for it. With a much longer line and ADSL2+ at my old home I only gained a couple of hundred kb/s from ADSL2+, here there's no option for it apart from the two LLU providers with a not-good reputation. This may be weird but I could slow down if I changed to the more modern tech.