I too have this issue and believe it to be the way the signal and sound are broadcast.
I also have a humax box and both are set up as you describe and have the same volume discrepancy.
I also have to change the volume significantly on my Bluray player dependent upon discs played (DVD and DVD audio discs have vast changes in their sound levels).
This was raised a few yearsago ago butI have flagged this for an OCE response relating to the youview box issue to see if there has been any recent updates on this, but I don't think there is anything that can be done.