I've seen the same thing- last year we had a series of power failures due to a weird cable fault that the leccy board had difficulty locating, and after each failure a speed test would show ~73-75Mbps. It would maintain that speed until any "normal" re-sync, when it would revert to the usual ~68-69Mbps. Then go back up after the next power failure
Only theory I could come up with was that everybody else on my cabinet was re-syncing at the same time, and that the cross-talk noise caused by that was lower than when all the lines were up and running normally. But the increased noise when everyone was connected wasn't enough to cause the DLM to throw its toys out of the pram.
But I'm open to more informed suggestions