@stevehodge, that's pretty much what I've observed too. A bit non-intuitive and I have little idea how DSL actually works, but maybe there's a constant low level of communication between modem and DSLAM and the error rate is only measured against that so it will be consistent regardless of usage?
On another note, my error rate has settled back down nicely after switching the target SNRM (Intended signal-to-noise ratio on Line settings page) back to what should be 14. Oddly it's actually stayed at 13db though - it's not unusual for it to drop to 13 during periods of the day in the past but it's now been sitting at 13 constantly since rebooting. Still, nice low error rates I'm just gonna leave it alone now!