Ah, thanks HP, I surmised there must be good reason.
You've cleared up a similar mystery; another chap had a master socket and a single phone extension cable plugged into it, run under the floorboards and emerging at the top of a huge oak sideboard as a loose socket into which was plugged a microfilter, then phone, and also ADSL cable to router. Router sync rate was (I recall) around 60kbps. But back at the master socket, the sync rate was 3800. Damaged/faulty cable think I, and we followed it back, bit by bit, to where it emerged from underfloor to the back of the oak sideboard. Whoops, there was the unseen extension cable drum - with a built-in microfilter - with a phone cable plugged into the phone socket (ADSL socket was empty) and leading to the top of the sideboard where another microfilter was connected. In that case I marvelled at how any ADSL signal at all got thru! But as you point out, the filter isn't 100% effective, and hence I guess there was just enough to get 60kbps.