*laughs* - yes, well, I would say if you fully understood how BT's DLM works, then you would be famous, as you probably also understand where the universe came from.
My line had a fault until it was fixed (a few weeks back). Broadband was still usable, but I was syncing with a 6db margin most of the time, with loads of FEC errors (I forcefully enabled interleaving, through AA's portal).
Magically one day, the DLM decided to raise me to 9db, slowing my sync speed. I contacted AA who got BT to reset it back to 6db. I also asked if they could set it to 3db, but the guy told me 6 was the lowest they could do, and that this was not fixed as the DLM may reset it again, if it decides to.
The fault got fixed. Amazingly, it now insists on making my modem sync at a 3db margin with a whole 1 mbit more speed. But hang on, I didn't think 3db was a valid target SNR?
In short, God knows how their DLM works - literally. I've given up trying to make any sense of it. As long as it doesn't annoy me and doesn't make completely insane judgements about my line quality and what speed I should be syncing at, I ain't too worried.