Regarding Winter/summer attenuation changes, take a look at the example from my connection from January.
I do see increased attenuation, lower attainable rates & lower sync speeds every summer.
Since I started logging my stats back in 2011, August has always shown the highest attenuation.
Also, remember that some stats such as QLN & Hlog only update when the connection retrains.
It is feasible that conditions can change fairly significantly between resyncs/retrains.
If SNR/SNRM have also reduced due to increased crosstalk and/or increased RFI (maybe increased 'skip' propagation etc.), it can effectively rule out any use of the next higher band altogether and/or only be able to use a few tones from it.