Hi
Quite a few VDSL lines will work quite happily at 3db, it's a margin after all, just less of one.
So something could have happened where you've sync'd back at a higher sync speed with the normal 6db margin, perhaps because of a wide area event like a power issue and you sync'd up first. As other people come back online cross talk increases and the margin decreases, but if your line is happy on 3db it will remain there.
The one way to test if you are on a 3db margin is to power recycle the modem/router and let it resync back up, if it remains at 3db then yes you have a 3db profile on an ECI cabinet, if it's back to around 6db, you didn't.
Regards
Phil