Certain figures are taken during the initial 10 days on BTw DLM, which are permanently stored.
Sync speed over this period is monitored and used to work out the
MSR and FTR which will stay the same for the whole time you are on a particular product ie adsl1, adsl2+ and also presumably FTTC.
This initialisation period is often confused as part of the everyday DLM process, but its NOT... as the DLM is a continual ongoing process. Its more of a one time only event, although it can be reset by request after Engineer work, it seldom is.
Any of the BTw speedtests where you login on the BT realm were also stored for up to 7 days, although not sure how often these are done now that the BTw performance tester has changed. These results could also be accessed by the ISP.