A few people say it's because the boxes don't alter the signal, they just pass it to the AV receiver. But then broadcasters are saying the levels are the same across different channels.
I personally wouldn't care if that stereo PCM SD signal got messed with on it's way to my reciever if there was a good reason (i.e. to not deafen you). Leave the HD signals alone, sure, those seems consistent across channels.
I haven't had the gear long enough to determine whether BluRays/DVDs/Roku will require different levels (although I can alter this on the AV input settings, I can't alter specific codecs from a single input).
My workaround has been to set the YouView to stereo mode set the AV receiver to something like "All speakers stereo" mode where the surrounds/centre/sub also get fed the signal. Fills the room a bit more than with just L/R doing the work whilst also engaging the surrounds for a faux-surround effect. This is only on the YouView input of course, BluRays will play directly as intended 
If only the HD channels could do that I'd have a perfect setup.