Broadstairs,
To me, knowing next to nothing about broadband, the interesting about such phenomena is to try to understand them,
without presupposing they are important.
If you use dslstats, it has a "bits" page, with a "botloading" subpage, where you can superimpose SNR per tone (under
"configuration > items to monitor > other". In my case, there's quite some correspondence between droopy lines in
the SNR graph, and lower numbers of bits per tone. It changes from time to time. I imagine the droopy bits, hence
under-preforming tones, correspond to sources of noise somewhere on my line to the PCP. Roughly speaking, the worst ones
(for me) occur smudged around tones 250, 1400, 3000 etc). These vaguely line up with with bitswaps per-tone, also on dslstats.
Of course SNR and SNRM are different, and AFAIK SNRM varies per line, and even per U/D "band". All these things vary
by time of day, how sunny it is, and whether or not it is the weekend.
Anyway, there's a mass of baffling stuff going on, that stimulates the imagination, perhaps enough to read some of the
"tech" xdsl tutorials on this site, and really look at the Dslstats and MyDsalWebStats graphs. The more I read and look,
the less I feel inclined to fire off a complaint to my ISP.
Mind you, I'd be very interested to read any comments you get from the kitties.