Openreach do not set the base figure in the modem. End of. How it is obtained is entirely up to the modem manufacturer, as you correctly point out.
The posters concerned know what they are observing and how to interpret it. They also know how their actual/attainable/SNRM behave when the Openreach base setting at the DSLAM is 6dB, and how that behaviour has changed.
You said it is impossible for the modem to be using 6dB as the base figure for calculating attainable. You cannot possibly know
that for exactly the reason you state.
Similarly, all the posters (including myself) discussing the matter are fully aware of the effect interleaving has on the calculation and that the result is usually crackers.
As has been stated, the only way to be sure of the sync-time DSLAM setting is to take the stats immediately after connection, or better still from the modem log if it records it.
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
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