No, that can't be it. Volume leveling is *designed* to work with files with all volume levels, not just files that are recorded low. That's why Volume Leveling reduces the volume of tracks with high average volume levels. The whole system is designed to work, transparently, without clipping.
I can assure you from quite a bit of testing (done while writing the volume leveling wiki article and beta testing the new volume leveling system in MC21) that volume leveling works the way I described it with files that have a positive volume leveling offset. You can easily test it by playing a file with both: 1) a positive volume leveling adjustment and 2) a true peak at or above 0dBFS; at 100% internal volumes, such a file will receive 0dB of boost (or might even get a reduction) from volume leveling.
This isn't really a design choice, it's a necessary consequence of digital audio: you can't get higher than 0dBFS in a digital system, so unless JRiver redefined 100% internal volume as something lower than 0dBFS there's nowhere for the boost to go without clipping on the peaks. You're correct that volume leveling is designed to work transparently without clipping, but sometimes the preventing clipping part means that you can't get perfectly level playback on all files when internal volume is maximized because some files need boost and there's no headroom to add it.
It used to be the case that JRiver never applied the volume leveling boost when the file had a near 0dBFS true peak because of the risk of clipping. I and others lobbied Hendrik to make it adaptable to when the internal volume was set lower so that it would scale up appropriately, and those files could achieve a level volume when there was enough headroom to accommodate them. That way one could permanently set one's internal volume to 80% or 60% (or whatever) and get perfectly level volume all the time, even with tracks that couldn't be "leveled" at 100% volume.
More info here:
http://yabb.jriver.com/interact/index.php?topic=99406.0I promise that's the way the feature works, and that's almost certainly why you're seeing the scaling as you approach 100% volume. If the file in question doesn't have a true peak that's closer to 0dBFS than the recommend volume leveling offset, then you might have found a bug. If that's the case post the volume leveling adjustment and the true peak, and maybe we can get to the bottom of it. Otherwise it's working as intended by scaling.
This makes a lot of sense actually. I *am* using a 2 channel playback system for these experiments, so I *am* downmixing from 6 channels to 2. So I did another experiment: I set up Null Output as my sound device. Then I re-ran the test. This time, I did NOT see the downmix. It said it was sending 6 channel audio to the null output device. But I was quite surprised to see that the output level *still* changes between 85% and 100%. The value from 5% to 80% is less now. But it still changes above 80%. This can not be how the system is designed to work. It makes no sense.
It makes sense if you listen to what I'm saying above. You can't boost a 0dBFS signal without clipping. A file can have a 0dBFS peak but nonetheless have a very low average volume level (such that R128 still recommends some amount of boost to reach the leveling target). Such a track cannot be boosted at all without clipping if you have volume set to 100%.
Similarly, a file that needs a +6dB boost to achieve a level volume, but has a true peak at -4dBFS, can only receive a maximum of 2dB worth of boost at 100% volume without pushing the peaks into clipping. Volume leveling knows this and only applies what boost can be safely applied, but if you lower the internal volume, volume leveling recognizes this now, and will apply more of the needed boost until it reaches the correct value.
The alternative is to never add the recommended boost when it could ever cause clipping (at any volume setting), which is a strictly inferior outcome as level volume is then unachievable (for those tracks) at any volume setting.
I tested it that way many times and all the video files are still all over the place, some low some high!
Can you post a little more info about your setup? What kind of volume settings are you using? Can you post the volume leveling adjustments and true peak levels for a few files that you think don't sound particularly level when played back to back? If you reduce internal volume to 80% do you get level sound?
These ones are never raised in volume to match the rest when using volume levelling and analysis....
For the reasons described above, the ones needing boost can't get it if you have internal volume all the way up, so if this is the case, you may just need to (paradoxically) turn down the internal volume to create the needed headroom to do all the leveling.
NOTE (post edited after initial post for clarity and to add examples)