The data is derived from BT and I seem to recall that the profile only updates when there is a change in sync speed, so the last set of data on the list should still be applicable, assuming you've not resynchronised, irrespective of the date (I could be wrong about that but I'm sure Azzaka will clarify when he's next on).
The Zen line data tool can be a very useful reference, and there's a bit more about it here.
We pull the info directly from BT systems and then publish it. So you are right Orainsear.
BT systems do have issues occassionally and this in turn is reflected on the data not being updated. As for the slow speeds it could be related to your exchange being upgraded to 21CN. Now i have been informed that the MSAN's and the DSLAM's should not interfere with each other nor the upgrade work should interfere with your line, however it still seems some lines are affected by the upgrades.
If you can PM me your details i'll be more than happy to check your line for you. Just to add, one of the reason we ask all customers to test using the BT Speedtester is due to the fact it stores all the line results in thier database and enables the ISP and BTW to pull the results when a fault is raised.
Kindest Regards,