As you know in large datasets there are always individuals that contradict the overall results.
Of course. I was hinting that you could use your own and other 'trusted' connections to attempt to ascertain if the median/mean results are typical across these trusted connections. Try to cut out the local connection issues (or point to them, if applicable).
With regards Plusnet, my guess is that the traffic management angle is part of it. Perhaps Plusnet doesn't suffer from a higher percentage of heavy users in the same way others do, and the habits of said users may create large amounts of night-time traffic.
I'm on a legacy product and from time to time, I have to consider after midnight for High Def or excessive downloads. Maybe the heavy users were 'trained' to behave in a similar fashion prior to unlimited conmtracts, and with PN's FUP they avoid PN.
If all BB providers publicly gave their capacity data, we could probably guess more readily.